The beauty of BD is that you can develop your characters any way you want. You can give a Wizard melee abilities and you can give a Warrior spell casting abilities. Picking the class at the start merely starts you with the stats needed for that class. From there on, you can assign points where you want them. You still need to be careful, though. You should make your character very strong in one, and use the other as kind of a back-up. In other words, make a powerful melee warrior who can cast some magic for a ranged attack to weaken your enemies till you move in for the kill. Or, make a powerful Wizard who can kill with powerful spells from afar, but who can defend himself should enemies be able to get in close.