Mages are as squishy as it gets and rely heavily on kiting. Kiting = mobility, which is the main problem for a boxer. So to be quite honest, I think mages are probably the worst class to multibox for pvp. Warriors could be worse.

That's why most people box shaman, because they're quite a tanking class in pvp. Heavy armor, healing and good on demand burst. Warlocks are somewhat similar if they got a healer with them.