Hey. I've finished this game like 25+ years ago and just bought it on GOG. I forgot about 90% of the game. Are Multiclass characters a bad Idea?

I wanted to make a Fighter/Cleric and a Mage/Cleric, both Half Elf. How about thieves? Are they crucial?

Also why my Fighter/Cleric can't dual wield?

First of all, there are two games in the series. Are you playing Strahd's Possession or Stone Prophet?

Second, although there are different ways to play, with only two PCs, might multiclass characters actually be a good idea?

Of course, it depends on what kinds of NPCs you can acquire during the game (and they vary considerably by game).

Personally, I don't think they're a bad idea. I love running around with a fighter/mage and fighter/cleric.
I like pure mages, but Strahd's Possession is rather brutal in the combat department. Later on you'll want access to lots of fire spells, but other mage companions can help out with that.

It also depends if you want to export your Strahd's Possession party to Stone Prophet.
Stone Prophet lacks any mage companions, making it imperative that you at least create a mage in the first game.
Stone Prophet also has a somewhat low wisdom Cleric (14; the other cleric is found much, much later in the game, with a wisdom of 16). At least this game is vastly easier than the first game though, thanks to enemies not running around like they're on crack.

Meanwhile Strahd's Possession has a bunch of mage companions to choose from, but is lacking in the cleric department.

Thieves are useless in these two games and Menzoberranzan.

I don't think clerics can dual-wield in old generation D&D games. It's against the rules.

For the Dreamforge/SSI games, I ALWAYS roll with a Fighter/Cleric and Fighter/Mage. :) Reasons are:

1. The level cap for the game is fairly low, so spellcasters typically cap out at level 5 or level 6 spells anyway, making single-class spellcasters a bit of a waste.

2. Spellcasting in the Dreamforge AD&D games is kinda wonky. As you do most of your fighting in cramped corridors, there isn't a lot of room for spellcasters to really shine when it comes to clearing out a room with a well-placed Fireball or trapping them in place with an Entangle or Web spell. (Indeed, most of these games don't even HAVE battlefield-altering spells like Walls and such.) As such, spellcasting is usually relegated to the role of healing, buffing and taking down dangerous enemies at a distance because missile weapons are absolutely CRAP in these games. This means that you'll generally be engaging enemies in melee a LOT, and only using spells against enemies that are immune or resistant to melee attacks, or that you don't want to engage at close range (read: level-draining undead).

3. Thieves are not terribly useful in these games. As far as I can remember, every single locked chest or door in these games has a corresponding key, so you're never locked out (hehe) of content by not having a Thief. The game engine also makes backstabbing a pretty lackluster combat mechanic, and there's no way for you to split the party, meaning that you can't have your Thief go off alone to scout ahead.

All of this means that you want two characters who are tanky and lethal at close range combat, but who also have the tools necessary to tend to wounds/poison/curses and blast dangerous enemies at a distance. That narrows it down to Fighter/X classes, and Fighter/Cleric + Fighter/Mage fits the bill the best. (I usually alternate between a Dwarven Fighter/Cleric and an Elven Fighter/Mage, or two Half-Elven characters of the same classes.)

I wouldn't say all ranged weapons are crap: The Axe of Hurling and Dagger of Returning can both be used to kill dangerous enemies from a distance, without the need to pick them up again (like arrows; bows are for this reason incredibly annoying to use).

You can often off enemies with them that are stuck behind a corner, but still partially visible.
They also really shine in open areas like the graveyard, where level-draining vampires can be taken out from a safe distance.

Because of his cleric nature he is limited to blunt weapons and maces and warhammers are too large to be dual wielded. You can dual wield a large weapon and a smaller one like a dagger or a short sword, but your cleric can't use them. You'll have to wait till you find the rod of smiting, which also is a blunt weapon and apparently small enough. Then your fighter/cleric can wield a warhammer rod combination or a mace rod combination.

Ahh, that's true. I forgot about those little wonders. I mostly just remembered what an absolute pain in the rear it was to use slings and bows and needing to pick up all of your ammo after each fight. XD

Yeah, it's dreadful. Same with the Eye of the Beholder games.
One would wonder why the developers back then didn't think about a system like Legend of Grimrock has, e.g. automatically picking up projectiles when you move onto them. They couldn't have enjoyed the ammo system when testing their game, could they?

Back then you had to pick up stuff manually. I'm not sure computers even had the ressources back then to track which item came from which character. At least they put them into their quiver if you double right click the arrow on the character's portrait.