2008 iMac how to use as a second monitor for a 2012 iMac – possible?

Hi all

Anyone know if it’s possible to use an 2008 model iMac as a second monitor with a 2012 iMac model? I called up Apple and they said no, but I can’t believe that this can’t be done. I appreciate any assistance you can provide It seems like quite a waste to have a perfectly working computer going to waste when I can be using it as a second monitor.

Thanks!