In pen-and-paper, once you switch classes from a monk or paladin, you can never take another level of that class. This is not the way it is done in NWN. What way is it done in IWD II?

Some time ago it looked like that, though I don't heared anything else so I guess it's still unchanged:

Even paladins and monks - depending on order - will be able to multiclass: Paladin and monk orders:
Paladins of Ilmater (LG): May multiclass freely to cleric and back.
Paladins of Helm (LG): May multiclass freely to fighter and back.
Paladins of Mystra (LG): May multiclass freely to wizard and back.

Broken Ones (LG): May multiclass freely to cleric and back.
Old Order (LN): May multiclass freely to fighter and back.
Dark Moon (LE): May multiclass freely to sorcerer and back.from: http://feedback.blackisle.com/forums/showthread.php?s=&threadid=29811