Hello Arathor!
Want to Backstab, eh? How about...
A Stalker?
An elven Thief/Mage?
A half-orc Fighter/Thief?
A halfling Assassin?
A dwarven Cleric/Thief? (backstab with club & quarterstaff)
Having Imoen / Yoshimo with you doesn't mean that you don't need a thief, it just means that you can concentrate on those things which the thieves in BGII ignore: pickpocketing, setting traps, stealth.
Or, you could just play a Sorcerer...
-- Nifft