There is no great solution to this problem. Therefore:

1. Avoid collection quests wherever possible, or only do them on your lead (this can be used while leveling under 60 to chain grant-a-levels to the rest of your team and can actually speed leveling significantly).

2. Beyond that, you basically have a choice between controlling from account A and then switching to loot on B, C, etc., or switching control entirely between toons. With a 2 PC setup as you describe I suspect the first option is going to be easier. That's actually better for you than a single PC setup since the alt account window you are currently looting on will stay active with the mouse cursor in the approximately correct location. Setting your camera angle to look straight down can be helpful as well.