I bound the Interact with Target on both of my accounts to 0. I bound the /assist macro to the - key on my slave. When I need the slave to loot something I just hit - then 0. simple.

easier still I use the N52 so I mapped those commands to the directional pad. On my main I do all the looting so I usually tell them both to loot so the main will grab everything and the quest items it already has it will leave for the slave to loot.