Sharing screens/monitor between Asus Laptop and iMac
I'm afraid that's not possible...

The closest you can get to that is by using Target Display Mode as described here. The problem, though, is that TDM is only available when running OS X on the iMac. Furthermore, you won't be able to use the iMac even though it's technically running. 

Judging from this question here at Ask Different, TDM doesn't seem to support multiple displays while running anyway.

I've looked at this before and did some research for this question, and I was unable to find anything beyond streaming the contents in an AirPlay-like fashion, which I doubt is usable in your case.