Warrior classes get an extra .5 attack at levels 7 and 13 only -- maximum 2 base attacks per round. Anything above that must come from other enhancements. Yes, THAC0 is stacked. If your warrior's base THAC0 is 2 and he/she wields a -3 weapon, then he/she effectively has -1 THAC0 for determining chance to hit.

The game is exp capped. That means you can have 161000exp in BG1, 2.8mio exp in BG2 and 8mio exp in ToB. Each class needs a different amount of exp for each level. You must plan your classes in a way that:
(exp for level x old class) + (exp for level x+1 new class) < (exp cap for your game)
When you get more than 2.8mio exp in bg2 you get an extra ability at each level up.

You must also know that you cannot use any skill of the old class until your new class has a higher level than your old class. And you will never get better in anything of your old class again.

My advice: Do not dual class in bg1. You will be unable to use the skills of your old class for most of the game. If you want to dual class, play a single class through bg1 and dual class at the beginning of bg2, when your char gets level 9 (fighter, ranger) or 10 (thief). You get max HP, most of the importand skills and you will get enough exp for the second class fast.

Of course, it must be human and you must regard the stat and alignment restrictions mentioned above.

@Hickory: You are correct. I think my fighters had 5/2 attacs at high levels (1 base attac + 0.5 at level 7 + 0.5 at level 13 + 0.5 weapon specialisation).

I don´t know how it is done with x.5 attacs per round. Does it mean that you have x attacs every round and x+1 attacs every second round?

I said it poorly. Is it not allowed or considered bad form to allow the points to accumulate (my example after getting a level 7 fighter). Then, when you have enough points, pick dual and level up to Level 8 Mage all at once. You would have a level 7 Fighter while the points build up.

Yes, the half attack is 'reserved' until it rounds up, which means that a warrior with 1.5 attacks per round will get 1 attack in round 1, and 2 attacks in round 2.

Don't the rangers get that extra 1/2 attack at Ranger lvl 7?
You only go to Ranger lvl 9 for the extra Proficiency Point for the weapon.

Mages get (1 - 4 + constitution modifier) HP from level 1 - 10. Starting with level 11 they get only 1 HP per level up.

Thiefs and bards get (1 - 6 + constitution modifier ) HP from level 1 - 10, They get 2 HP per level later and they level up faster than other classes

Priests (cleric, druid, monk) get (1-8 HP + constitution modifier) HP from level 1 - 9, then 2 HP per level up.

Fighters, paladins and ranger get (1-10 HP +constitution modifier) HP from level 1 - 9. Barbarien get 1 - 12 HP instead of 1 - 10. Unlike other classes they profit from constitution above 16.

The best (at least from my point of view) classes are fighter -> mage, fighter -> thief, ranger -> cleric

By taking fighter or ranger 9, you get maximum HP, 1/2 attack and a weapon profiency every 3 levels.

Be careful with weapon profiencies. You wil lose those of your old clas when you dual, but you get them back when your new class gets better with your old class.
example: you start as fighter and put 2 points to longsword. Then you become a thief and put one point in longsword. When your thief level gets higher than your fighter level you will have 2 points there and you lost one point.

I have found my Kensai to be the best character I have ever created. Using no ranged weapons may be a little difficult, so at the beginning of the game I have him sit back and relax while everyone else fires the long ranged weapons. Once the baddies get close enough for melee, then I let him have his turn. He's a level 8 currently and is a real badass. The first 3 levels were a little of a challenge, but after that no problems. Just remember to have him defend while using the ranged weapons.


****SPOILERS****

Stole one of Drizzt's scimitars and then found another in Durlag's Tower and now he is a dual wielding scimitar killing machine.
Another tip for people, I noticed it's best to put two specializations in his weapon class and two in his dual-weapon specialization. I tried the other combinations 1 in weapon and 3 in dual and 3 in weapon and 1 in dual, but the even 2-2 was better for his THAC0.

You can kill Drizzt and get both swords and his armor.
It will be an extremely hard fight if you play fair.

Stun him with a spell and finish him.
As a drow he has high spell resistance and he also has good saves.
After some reloads I was able to kill him while he was helpless.

Killing him will lower your reputation.

If I was playing an evil party I would have killed him, but with a good party, I'd rather not. One of these days I'll go evil and then that battle will happen.

I hate him, so good or evil, I always steal his sword, then kill him. Just a personal quirk.

I used the character progression charts here when I created my Level 13 Kensai/Mage. As mentioned by Mad3, you lose your original weapon proficiency points when you dual to a new class and they do not stack with already chosen weapon proficiencies of your prior class.

As I was a little obsessed about not losing proficiency points, I held of leveling up my kensai/mage once I hit mage level 11 so as not to be forced to choose a weapon proficiency until I hit level 14 (which would then enable my Kensai class and allow me to increase my reactivated weapon proficiencies); an exploit, but like any tweaked Dual-class characters, sacrifices must be made; hence a high-level dualing is definitely not recommended for a first time playthrough. Note that the Mage progression table is not quite correctly implemented in vanilla BG2 and you will lose spells (specifically with a dualed level 13 fighter of any class you will miss out on the final level 9 spell slot at Mage level 27), without using mods.

Most people (including myself) that do go for the level 13 dualing probably do so for the extra 1/2 attack gained at this level; however it is worth noting that there are other benefits especially with a specialist fighter class. First off we have a significant improvement in saving throws; saving throws from multiple classes do not stack, it simply uses the best save from what ever class gained it, however both the Fighter saves and mage saves combine together to give you some very strong saving throws.

At level 12 for a Kensai you gain your final bonuses giving you a total bonus of +3 to Hit, +3 to damage; -3 to weapon speed factor (not as useless as it might seem, I use the 'answerer' long sword on my character which has a very slow speed factor of 9 so it hits a bit quicker then it normally would otherwise (9-3 gives us 6), -2 bonus to AC and 4 Kai attacks per day over a standard fighter class. With that +3 to hit bonus a Kensai 13 hits the same as a Level 16 Fighter but with an additional +3 damage per weapon.

I want to put an important note here though, as Kensai's cannot use gauntlets, they miss out on arguably the best fighter based item in the game the "Gauntlets of extraordinary Specialization". This is one of the primary reasons others recommend the Beserker as an alternative to the Kensai (It may also be much better suited to yourself, as you can wear armour all the way through BG1). with the Beserker rage you get a bonus +2 to hit, +2 damage and a -2 AC as well as immunities to charm, hold, fear, maze, imprisonment, stun and sleep. He also gains 15 hit points while enraged, for a duration of 60 seconds.

With a dual-wielding Beserker wearing these gauntlets you will be granted an additional 1/2 attack a round. This extra attack means that you are hitting 8 times every two rounds (2 attacks/round for any fighter class at level 13, 1/2 attack for using a weapon you have specialized in, 1/2 attack for using the gauntlets and 1 attack for an off hand weapon). vs 7 times every two rounds for the Kensai. With the +1 to Thac0 and +2 damage also granted by these gauntlets, you will be, for a duration of 10 rounds be doing much better damage then the Kensai (+3 Thac0 +4 Damage & extra 1/2 attack/round).

When not in a beserker rage, you still will have the benefit of always attacking 4 times in one round, vs 3 for the kensai (4 on the second round), although the level 13 Kensai will get there bonuses full time as well as being able to use Kai to maximize damage for 10 seconds; it is quite possible that the Beserker may turn out to be a better damage dealer.

It is quite possible to enjoy most of the Baldur's gate 2 core campaign with a high level dual classed character; it just requires reducing your party size and taking on every single quest that is available to you before heading on to spell hold. I kept my party to 3 members including Viconia (Need healing!), Jan (Traps and locks) and myself. A very challenging play through considering my alignment being good and reputation really grated on Viconia's nerves, but my low level mage spells were not completely useless and you gain a good deal of experience simply learning new spells so keep all those scrolls ready for the mage transition. Once I hit level 14 mage, I gathered a full party and headed onto spellhold finishing BG2 with a Level 13 Kensai / Level 18 Mage (full party since spellhold).

The only dual class char I had so far was a berserker9 -> thief. I played through vanilla bg1 and used all books on me. He could kill almost everything by backstab, run, hide and repeat.

I don´t think I will make a dual char past level 9 of the first class. it takes so long until you can use your old skills again. I always play with a party of 6.

The only char who finished bg1, bg2 and ToB was my swashbuckler. His Thac0, AC and damage is better than a normal thief and with his fast levelling and the "use everything" ability he was fighting with crom fayr in one hand and another good weapon in the other, wearing full plate and all kind of items that were for other classes only and the gauntlets that give extra attacs. This way he had more HP, AC, damage and even Thac0 than a fighter with the same exp (again, thiefs level up faster) and almost as much attacs. When I removed his full plate, he could scout, take care of traps and locks and fill the whole area with spike traps.

The main reason is that most other chars failed at draconis. How can you defeat a dragon that disappears permanently before you can hit him? Even true seing did not help much.

While I like bg1 and 2 very much, I found ToB quite boring (except the tower). It is nothing but a chain of epic battles and the story and atmosphere somewhat dropped. At least thats how I felt sometimes.

If you are killing him is there any reason to steal his sword first? Is it for the satisfaction of killing him with his own sword?

I just like to steal from him first. The rest of his gear, even though it's awesome, I leave to rot with his corpse.