For two-box, you probably won't have the dps to drop things before they close to melee.
I like a healer type who can add some dps, and a melee type who can handle the mobs.
You could go with a BM hunter and another ranged toon if you like. That's basically the same thing.

With only two toons, it really doesn't matter what you pick; there are only two toons to keep track of.

Generally the healer/support dps will be the background character, while the melee/more active toon the primary character.
It's just easier to run like that.

A shaman (totems, the odd lightning bolt) makes a great support toon; if they go Resto they give E.Shield to the primary and use W.Shield themselves.
A dreamstate druid can heal very effectively, and provides probably the most dps for a healer spec.
Paladin/Priest spec'd as a healer won't provide as much offense as either of the other two, but you could spec them a lot of ways.