IWD2 uses the DnD3 rule system while IWD1 uses the DnD2 rule system.
In DnD2 you select a class or class combination at the beginning and the only way to change your class during the game is to dual class (If you are human and have some requirements, such as minimum stats)
DnD3 is completely different. You select a class when you create a char and every time you level up, you select what class you want to have. Your only restriction is alignment and some restrictions for monks and paladins. In theory you could have a char who has levels of every class that are allowed for his aligment. So the question is not what is possible (answer: almost everything) but what makes sense.
Think also about favoured classes and exp penalties.
general rule: If you fight with weapons, multi classing is often good. For example taking some levels of fighter to get feats and profiencies.
If you play a caster, do not multi class if you want your spells to be powerful.
Here
http://www.gog.com/forum/icewind_dale/quick_guide_to_character_class_ability_rolls_and_party_creation/page1 are some discussions about this.
I do not know IWD2 good enough to advise a char for soloing, but I will use a party of 6 for my first playthrough.