As the Imp said, a dual class (or multiclass) combo requires an actual class entry (FIGHTER_DRUID or whatever), which you would have to create. That stuff is hardcoded, though we talked about externalizing it in theory. Dunno if that ever went anywhere though.
The GemRP has the ability to add classes into it, but using another engine to play the game is not really ... where most peoples that can take a "No" as an answer, lean towards to. Especially considering that you yourself have to build the class code.
The farthest expansions the official games went to, was to make hte monk, mages and sorcerer classes to have kits in the EE games, and added the "druid sorcerer" -Shaman class, while the barbarian was subbed as a fighter kit.
Yep, Jarno Mikkola. my Mega Mod FAQ. Use of the BWS, and how to use it(scroll down that post a bit).
OK, desert dweller, welcome to the sanity, you are free to search for the limit, it's out there, we drew it in the sand. Ouh, actually it was still snow then.. but anyways.