You could just go up to a quest giver, get it on your main and then it says "Alt #1 already is on the quest" "Alt #2 has accepted your quest" and "Alt #3 has already completed your quest". Now if your main has already done all the quests for a zone or a large part of it, then act like one of your alts is your main for a while. Or just go to your alt and have it pick up the quests, it should still share it with everyone. Also, IWT (Interact With Target) should let you select a quest on everyone and only the quests that match should be picked up. Again, that will probably depend on who you have as your main. So your main picks up "Pick frood off ground" and the alts have already done that, they see on the quest giver "Eat the food", they shouldn't get the eat the food quest since it doesn't match the main's choice of get the food.

Short version, use the char that is farthest behind and have him act like yoru main and get/share quests until everyone is even up.