It was the Asgardians who thought that having more than one in the same place would be dangerous, and Tivan just played along. He WANTS to collect them all. He is the ultimate Pokeman player. Remember, "One down, five to go?"Here's what I don't understand: they bring the Collector the Power Stone in Guardians of the Galaxy. If you shouldn't have two infinity stones in the same place, why does the Collector allow the Aether and the Power Stone to be in the same place? Or should we assume he shipped the Aether someplace else when he knew another Infinity Stone was coming?