Well, Kensai dualed to Cleric might be able to wear while Kenai is inactive (never tried), but you get all the penalties when it is restored (when Cleri levels are higher).
Thac0 isn't easy as it is backwards. AC 10 is worst AC (this AC value minused from your Thac0 to see if you hit). The lower the Thac0 the better (it starts at 20 and counts down).
Remember that Str bonuses, magic gear, spells etc make your effective Thac0 lower.
With Thac0 20 vs AC 10, that means 20-10 =10 needed to hit on a 1d20.
AC 0 means 20-0 = 20 needed on a 1d20.