With true dual-boxing I find it best to do every single quest you can find. Collection quests still aren't that bad (as opposed to 5-boxing*), and a lot of the quests you find will give group credit. This happens even more in BC, where I find my Pally getting credit for everything my Warrior does. I had to delete an entire bag of quest items because they were never used.

If your characters are unique looking and different classes you could get invited to a PUG instance group (or start your own). I've pulled it off, and the other 2-3 people never knew I was boxing. It even adds an extra layer of challenge when you talk through 2 or more characters as if they were separate people

*Note on 5-boxing collections: Someone mentioned this before, and even though it is very boring/time consuming, it gives you a "grinding gauge" so to speak. If you have to collect 30 flying debris x 5 toons, then you'll really just be grinding for 5 times as long (possibly longer), which in the end balances out the XP penalty. Although, I believe anyone here would recommend instance grinding to world quests any day.