Out of Druid, Warlock, and Pally

[blizefy]

What is the best combo for a 2-boxer. Also, what should their specs be?

[algol]

There is no best combo for a 2-boxer. Especially when they say nothing about what they plan to do with the team, and don't seem to have read any of the mountain of relevant information here yet. I suggest you start with the wiki, possibly this page.

Those three alone give nine combinations. I'll tell you up front that melee-melee is a lot of trouble, which should rule out a few spec combinations if not necessarily class combinations.

[Ualaa]

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.