Generally a 2 vs 1 is going to come down to a matter of action and spell efficiency.
Without giving a ton of effects, the fact that two mages have 4 spells vs her 2 each round before familiars and spawnpoints and such, and the ability to coordinate makes it pretty impossible on ones own.
Having the ability to generate that many creatures, and that you have to go through double the ammount of key cards.. such as dissolves and dispells (the solo mage runs 2-3, but each of the other mages have 2-3 most likely meaning they can keep the solo mage unable to really equip or buff or debuff), or battle fury (each opponent having 2 copies per book means the solo has to endure a lot of damage)
The solo mage would have to have very powerful spells and effects to be able to generate any kind of tempo against 2 coordinated players. Ive tinkered with the idea as well, but you pretty much have to invent an "archmage".