I just tried to make a Thief dualled to a Necromancer by using ShadowKeeper, and I ended up creating a multi-classed human
Thief/Necromancer. I didn't know that was possible. Sure enough, he can't use spells from the opposition school of Illusion. He apparently has the same number of spell slots as a single-class specialist Mage. I think the reason why he's a multi-classed character despite being human is because I didn't dual-class him by using the dual-class function in the game. Since I specified two classes with ShadowKeeper, it makes him multi-classed by default. I'll have to remember that if I ever want to play a human multi-classed character.
Sorry, I don't know how to give a dual-classed character extra spell slots (without giving him a special item).
I don't know much about D&D rules. When I play Baldur's Gate, I play by Baldur's Gate's rules. Most of the time, anyway.
So here's my question. If I made Imoen a Thief dualled to a Conjurer in Baldur's Gate (which is what I actually did because the game allowed it), then why I shouldn't I make her a Thief dualled to a Conjurer in Shadows of Amn? That's what I usually do.
I think it's important for PC characters to have a past, whether the game takes it into account or not. Take a look at the NPCs, both joinable and non-joinable. The ones that are most interesting have interesting lives and do things that are out of the ordinary. Look at Jon Irenicus. Even if you had a 31st Level Mage, could you do the same things that he does--building huge labs, becoming the new director of Spellhold, creating golem armies, etc.? Obviously, some of those things are outside the normal rules of D&D. Or look at Nalia. She's really a Mage who learned some thieving skills when she slipped outside to help the poor. To take that into account, the game made her a low-level Thief dualled to a Mage, but that's not really
what she is when you take her biography into account. You can play her as a Thief and let her wield Thief weapons if you like since the game allows it, but I think it's more appropriate to take her unique background into account when playing her. The same thing applies to PC characters.