I was thinking about playing a Kensai to level 12 and dualing to thief/mage. When I get the 2nd class to level 13, can the Kensai wear bracers/leather armor now to improve the AC?

No, the Kensai can not wear bracers or armor in the new class. You can only use rings, necklaces, and cloaks. You can't even use helms, but you can use ion stones.

I would personally try a Swashbuckler/Mage (I like having a low AC). You get weapons, armor, bracers, and can still cast stoneskin, tenser's transformation, etc. You'll get decent thieving skills as a start. You'll sacrifice some THACO, but not a lot. And hit points won't be that great -- but they're still better than a straight mage.

Also, level 12 is a bit too late to dual class. Think about how much exp you have to get before you reach level 13 mage. Quite a lot. Until then you are a mage who can't wear bracers and can't even use a sling. Compare with the number of exp points you need at 10. Dual class at 9, that's the ideal.