If you want to "duel" him to a mage, I think it is easier below tenth level (they get those darned protection from non-magical/magical weapon spells.
If I dual to mage, I usually do it earlier in the game than the general consensus. You are not going to send your best mage wading into every melee are you? Leave tanking to the specialists. A good mage is best put to use as artillery anyway. What are the REAL benefits of having a mage that was a fighter? Hit Points, use of a sword (there is a Katana in BG2 that gives a spell bonus similar to the BG1 ring of Wizardry) and the ability to wade into the occasional fight if needed. Sure you can use elfin chain, but do you need it? A 5th level fighter maxed out on CON should have around 50HP. Add points from a familiar and 70HP base mage is not bad. If I create a new BG2 char, I dual right away. If you are in BG1, then you might already want to dual.
-Cayle