Well it seemed like a good idea at the time - using my old 27" iMac (2010) in Target Display Mode as a second monitor for Cubase pro 10 which is running on my newer (2014) 27" iMac. But, after 3 conversations with Apple support, all with conflicting answers and endless searching on the interweb - I am still not sure if this is possible.
Issues are cabling - Thunderbolt to Mini Display Port - and exact iMac versions. Target iMac machine is Mid 2010 (which Apple seem to think is OK) and main computer is Late 2014 - which according to one Apple support person is no good for sending the signal - just a little bit too modern.
SO - and I know this is a computer problem and not a cubase issue - but I figured some Cubase users may have some real experience with this and have got it to work. It would be nice to be able to do it.