I have two 21.5 iMacs (Mid 2011) and Id like to use one as an external display. I have a mini displayport to mini displayport cable connecting them. Ive read that this only works with the 27 iMac, but have also read it works with 21.5 iMacs with Thunderbolt ports (which my iMacs have). Either way, my second iMac isnt being recognized by the other iMac. It may be because my cable is mdp to mdp and not thunderbolt to thunderbolt. I'm running Mountain Lion 10.8.2 on both computers. Any suggestions?