Mini DisplayPort to Mini DisplayPort and Target Display Mode

I wonder if any one else has encountered my problem . I have a 27" IMac Mid 2010 and a late 2012 iMac 27" and want to use Target Display Mode on both. I can get TDM to work from the newer Mac so that is the main screen and the older iMac is the secondary screen but as yet TDM wont work the other way round so the older IMac is the main screen with the newer mac as a secondery screen. I do a lot of photography and all my photographs and software are on the older IMac so been able to use the this as the main screen would be extremely useful.

I couldn't find a genuine Apple Mini DisplayPort to Mini DisplayPort cable so I bought the Belkin one which as I say works only one way using the newer iMac as the main screen. I know the Mini DisplayPort itself is working as I used to have another external monitor that worked using the Mini DisplayPort to VGA cable.

Please does anyone have any advice or answers.

Neil