I would think if you want to use shot rotation effectively, making hunter main would make that a lot easier. As a 70 hunter, I tried dual boxing two hunters, my own and a friends. While rotation on my own worked like a dream, even with the same gear (guns, ammo & pouches) the follower soon lost sync.

That could be in part due to latency, my own is often too high to be much fun. But its something to think about.

The other thing to think on, at 70 my pet can't keep aggro from me, unless I FD pretty often (21/37/3). If the idea of dualing these two is to have the pet tank, the Mage will have to lower its DPS by quite a bit to keep the hate down. Or the pet will lose aggro. And while I'm not a BM Hunter, I play with one regularly (better gear than me too) his pet is a better tank than mine, it lasts longer and keeps aggro better, but it can't keep the agro off a high DPS mage.

((Note: Most of the above is based on single boxing with my guild.))