You cannot multiclass if you are a human not at all.
Yeah and that was a good thing. It made humans special compared to anyone else. In 3.5, humans could multiclass any class without getting an XP penalty. Dual classing was also actually fun, and given that I still play BG2, I'm a big fan of the thing. I also liked the idea that certain classes demand such dedication that you can't multiclass, like the paladin.
BG2 may not be perfect but lets not forget that its a great game and fun to play to this very day and I love it a lot.
That being said I prefer 3.5. It's my favorite edition, and we will just have to wait and see what happens with BG3.