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.)
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