Can I use a 27" iMac as a additional monitor for another 27" iMac?
What you're looking for is Target Display Mode, built right into the iMac. Apple describes it in full in their knowledge base article.
- Yes, you can use one iMac as monitor hooked to the other iMac
- Yes, you can still use both machines at the same time
You can see these two questions answered in this video: http://www.youtube.com/watch?v=oHlmGSQr53s
- No idea :-)