i plan on doing the kensai/theif thing and playing it threw ToB. what level should i dual to thief so i can get to maximum thief level. someone i talked to said it was 13 but it seems to me that it should be higher. can anyone help ?

I believe u wont get to the max thief level if u dual coz ur first class will surely get some of the levels. Anyway In my opinion the best time for a kensai to dual is at 12-15. Youll have a pretty good Thaco, at least 3 kai, and u dont have to wait that long to activate this class after being a thief.

Since you have ToB installed, dual into your thief after your Kensai hits 13th level. You get one extra attack per round. If you are dual wielding it takes you from 7/2 attacks to 9/2. (That's 7 attacks per two rounds and 9 attacks per two rounds.)

With ToB installed, you can definitely achieve your maximum thieving skills. You might not reach 40th level, but you'll be up there.