Actually, I'd disagree with you on this count. If anything, spellcasters in this game seem to parallel heroes in Warcraft III, in that heroes were the only real counter to other heroes. In that game, the advantage of two players starting with two heroes, as opposed to one player starting with one, was such a huge edge that even very experienced players couldn't realistically challenge two reasonably proficient players. We don't know yet enough about how this game plays to see how it will turn out. It's too early to lay judgement on this matter.
Well, seeing as how wizards will actually gain better spells as they level up and gain more experience and all, I think that two wizards that are half as good as the third one, combined, either are just as good, or worse than the third, experienced, magician.