The main advantage about the Kensai is their impressive THACO and +damage, right ? And the main disatvantage is their usually 'low' AC (which is, literally, a HIGH AC *g*). So, could you dual a Kensai to a Berserker or alike ? If you can, would this do you any good ? Like keeping the awesome THACO and still wearing armor ? If Fighter is not possible, what about cleric/thief/ranger ? I'm not very familiar with the dual/multi-class-rules, as you can see

Nope. That would be having your cake and eating it too. And you can't dual into kits, only true classes.

You can't dual a fighter into a fighter... heh. And when you dual, you have to obey by the rules of both classes. Sort of. Well, it's not as strict as multiclassing, but to put it simply, no matter what you dual a kensai into, you're not going to be using armour (unless you count a robe as armour, and dual to a mage).