I have two 27inch iMacs, both 2011. When I connect iMac A to iMac B and let it sleep, iMac B's display does not wake up. However, it still shows up in the Displays and I can still play audio through it. When I connect iMac A to iMac B to use as an external display, they both wake up after sleep. Why does it work one way and not the other?