I never realized how customizable so many of the BGI NPCs are! There are 6 NPCs you can legally dual-class in this game!
Dynaheir, with her 17 INT and 15 WIS can become and Invoker > Cleric with 2 WIS tomes.
Imoen, with her 18 DEX and 17 INT can become a Thief > Mage.
Branwen, with her 16 WIS and 16 DEX can become a Cleric > Thief with the DEX tome.
Safana, with her 17 DEX and 16 INT can become a Thief > Mage with the INT tome.
Shar-Teel, with her 18/58 STR and 17 DEX can become a Fighter > Thief.
Xzar, with his 16 DEX, 17 INT and 16 WIS can become a Necromancer > Cleric with the WIS tome, or a Necromancer > Thief with the DEX tome. That's versatility!
Now, I know most of these tomes can't be found very early on, so you're kind of limited with the timing of everything. A quick journey to Durlag's Tower will enable Xzar to become a Necromancer > Cleric if you wish though, and both Imoen and Shar-Teel don't need tomes.
I always thought BGI had so many possibilities with all of those NPCs out there, but now I see even more possibilities with the fun tool of dual-classing!