[Cerek the Barbaric]

Actually, while looking at another BGII HELP site, I saw a section on THE BEST CHARACTER.

One suggestion was to choose the Wizard Slayer class (which I had already done). This class develops good Magic Resistance as they gain levels. Play the W.S. to 15th level and the dual over to a thief.

You will be able to reach something like 32nd level as a thief. You won't gain any of the warrior high-level abilities, but you do get the absolutely best ability USE ANY ITEM for being a thief. This means that your 15lvl fighter/23rdlvl thief can now use the HOLY AVENGER - one of the strongest weapons in the game.

So...the answer is that it is NOT too late to dual-class, but the thief class is probably the only feasible option. Of course, you will also get to set all of those nifty Special Snares.

Keep in mind that you need a 17 in the primary stat of the class you want to dual to,

Personally, I chose NOT to dual-class on my first trip through. But I have Saved a copy of the game with my character at 15th level so that I can try it later.