So I'd like to use my old 2009" 27" iMac as an external display for my newer iMac. I assume this is possible, as I've seen people do it with a macbook. But, when I plug it into my iMac, nothing occurs. Is it because I'm using a thunderbolt cable instead of a mini display cable? If so, and I buy a mini display port cable, how do the computers decide which is the computer and which is the display? Thanks!