Do all your normal quests first. I think if you hit 70 in Blade's Edge, doing all the Netherstorm+Shadowmoon quests nets you ~3k per character.

The three Ogri'la dailies (not the simon-says one) are groupable, so just doing it "once" nets you 12g per character. 2-boxing I can do all 3 in about 30m, with 5 that would be 5*12*3 = 180g in half an hour or less. The Netherwing ones are great as well, doing 7 of those would net you 5*(6*12 + 18) = another 450g. Those take me ~2-3h 2-boxing. That's also not counting all the drops you get along the way.

Farming older instances is profitable too, Large Brilliant Shards sell for ~15g on my server right now, and I'd imagine you could get at least a couple dozen in an hour doing strathome/scholomance.

The most restrictive factor in multi-box farming is respawns. You can kill things so fast that you spend most of your time finding stuff to kill. Instances make it easy to have a non-stop stream of npcs you don't have to fight other players for .

Farming nodes and AH is decent, but doesn't take advantage of multi-boxing at all.