Using iMac as screen and a file server?

My boss recently bought a new rMBP, and since then has been using his old 27" imac as a secondary display. We were wondering if it would be possible to also use the iMac as a local file server running OS X Server, while also being used as his secondary display? I wasn't sure if there would be a way to select what input the iMac screen would use or anything like that.

I expect it's impossible, but I figured it's worth a thought.