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).