Mid 2010 iMac as a second monitor

I've got two iMacs and I want to connect the second(older) iMac as a second monitor. The problem I'm having is that neither of the iMacs will recognised each other! The iMac that I'm working from is a 5K, 27-inch, 2017 and I want to mirror to a 27-inch mid 2010! I have tried using three different thunderbolt cables and I've also tried using the cmd + f2 function. Any help would be greatly appreciated!!