It has to do with ability scores as well as design. The game won't let you dual some of the NPC's. If I remember correctly, Imoen is the only NPC in BG I you can dual class. But I could be wrong.
Edwin, mighty founder of the Council of Mages; Co-founder of the Shadow Mages
Sorcery and Shadow together as one, the arcane and the dark united. Through our knowledge and skill none can stand against us. We are as one, infallible and invincible. The Shadow Mages.
You need 15 in your Prime Requisite (the stat most important for your class) for your original class, and 17 for the class you're dualling to.
[url="http://www.sorcerers.net/Games/BG2/SpellsReference/Main.htm"]Baldur's Gate 2 Spells Reference[/url]: Strategy, tips, tricks, bugs, cheese and corrections to the manual.
I cheated and raised Shar-Teel's Intelligence to 17. After that I was able to dual-class her to illusionist. What a mage! Wears a helmet, handles a sword and uses a bow! And Shar-Teel looks stunning in a robe ...