You get slightly more than half the exp per kill, but kill almost twice as fast. This makes quests that involve killing monsters go faster, and leaves other quests at the same speed. It is faster leveling two together than separately, but as said, it is probably faster leveling one solo if you only want one(which the op doesn't, but others viewing this might).

Note that looting quests (like kill/loot 10 troll ears) will be faster leveling together since if you level them separately you are still doing the gathering quest twice(bring 10 troll ears with two mages at a time for 20, or bring 10 troll ears with one mage, then tomorrow bring 10 troll ears with the other mage).

Something that I'm not sure about is leveling one mage by itself, then using that one to power level the other one. I'm guessing this isn't faster for two, but for someone leveling 5 it might be.