I have a late 2012 iMac and a mid 2011 11"air. I just got myself a thunderbolt to thunderbolt so I can use my air as a second display port. I read that after plugging in both ends to each computer, I have to press command + f2. I get no results. I have tried it on both and I have received no signal. Is there anything I could be doing wrong? i am not using an apple keyboard/mouse, if that matters in any way.