I only dual box, and most of that is a high level with follower. As someone who doesn't enjoy end game grinding or instances much, I get 90% of my gold from grays and other instance loot. I find that with one of my alts being an enchanter and the other an engineer, I can pop down a repair bot with the engineer and sell grays without leaving the instance, something that is rather brilliant for those instances a long way from vendors. And while the bots do cost a lot to make (though I think they cheapened the lower one in 2.3), 110g approx for the high end one They are more than worth their weight in gold IMO.

At the moment its even easier as my main follower is an enchanter/tailor, so I can really stack in the loot; food, water, grays, whites... etc go to the bots; greens, BoPs and any other DEable loot gets DEd then sold; and the cloth gets turned into Bolts of cloth which really compresses things.

As for looting itself, my main Loots everything generally, if he needs to mana/hp up (hunter) the alt loots while he rests. It seemes to work out quite well. And bag space is roughly this. Hunter has first bag with pots/engineering gear/explosives/mounts/etc, and the last bag is ammo. 'Lock has one bag for gear and pots and a shard bag. Hunter uses some 18 slots and 'Lock has 16 slots.

When I genuinely dual-box, not as often as I'd like but I get bored , again I use FFA and loot everything, only difference being that I tend to drop/not-loot grays.