i just want to verify if this will work.

Im going to make a human Archer. Play him right through the game once and get him up to maximum Experience. Then import him back to the start of the game and dual class to a theif, this would be perfect as i hate all the Kits for theifs. Then play right though to maximum experience again and ive got the perfect character for throne of Bhaal will this work?

No it will not work, since you cannot get more experience..
So you'll be stuck with a max level archer and a level 0 thief ;)

huh?i thought after you dual class you get a new xp cap?

Nuhuh! When you dual class you only start at level 1 and get the difference of your first class and the XP Cap to get for theif.

Ummm rangers can only dual class to clerics, sorry.

No, you don't get a new exp cap after you dual class, but you can remove the cap. It doesn't let you go past level 40, but you can have a level 40fighter/40mage/40cleric multi class for example.

... What? No love for the bounty hunters? :(

no i dont like them because they get a penalty to theif skills. Anyway im thinking of playing as a theif now. Ive never done this before so give me some advice