I have an iMac from 2011 and there is already one external monitor connected via minidisplay port to dvi adapter. Today I wanted to connect another one, as the iMac features 2 minidisplay/thunderbolt ports. I bought a second adapter and hoped it would work just as easy – plug in and done. But the screen of that monitor stayed black, nothing happened at all. Do I need another adapter? Some additional software?