I'd suggest going with skinning on the secondary toon, you can macro.. "/targetlastenemy" and "/use skinning(apprentice)" or whatever skill your toon currently has, as a two line macro.
If your second window is large enough to view the minimap, give herbalism to one toon and mining to the other. Old world herbs and minerals are both selling very well, as most people would rather play WotLK then farm mats in the old world.
Enchanting is a good skill to have, if you'll eventually do instances... especially if you eventually go with a 5-man team.

The toons that level best solo, are also the toons that level very well as a 2-Box team.
Hunters (usually BM spec) can generally level faster then any other toon, but they are being heavily nerfed in the next patch.
If there's any possibility of moving towards a 5-Box team at any point in the future, the Paladin is the easiest tank to control as a boxer.
I'd recommend at most one toon that does its stuff in melee range, with every other toon on the team as a caster or ranged damage dealer.
A strong option might be a damage dealer/tank toon, combined with a damage dealer/healer toon - Tanks and Healers are the two hardest components to find for an instance down the road, by playing both even as a two-box combo, you'll be able to instance just about whenever you'd like to.
The shaman has a lot of synergy with most other toons, and could be a good healer. Whichever healer you go with, you'll want a toon that can also funtion as a damage dealer.