Could a ranger duel to a thief? I hope so

I don't think you could dual to a thief. You can certainly dual to a cleric as long as you have 17+ WIS, and possibily a mage with 17+ INT but not a thief.

EDIT: I've just checked and you cannot dual to a mage. The *only* class that a ranger can dual to is a cleric.

If I am not completly wrong, you may only dual to classes, which are also allowed multi-classes. Thus, a ranger/mage is not allowed, because a demihuman also cannot get this combination. And of course you must have 15 in the prime requisits of your first class and 17 in the prime reqs of you second class.

(At least this are the rules of AD&D PnP 2nd Ed.)

And an Archer->Mage would be incredibly overpowered, anyway.