Don't alt your melee character. Rather macro your mage so you can work your mage while maining your melee character. :P

It's much easier to get the max out of both chars that way.

Either way, to dualbox, nearly anything is possible. AoE fear won't mess you up too badly, it's not too hard to get two chars back together. So option for PvP are much more open as well. 2v2 arena would be nearly impossible, but as a dualboxer, you can sure be effective in a 5v5 team, or maybe even a 3v3 if the second player is good on the cc. In bg's, anything is possible. :P

Some dualboxes I've seen that sound like fun to me on an overall scale (pve/pvp):

2x boomkin, 2x mage, warlock/priest, 2x hunter, 2x rogue

More pve combo's:

resto shaman/tank or priest/tank (If you go resto shaman and prot warrior, you might be able to do decent respeccing your warrior for pvp as well. They have good synergies.)