So yes it works as expected. But it seems that now because I've changed ports I've got other problems. On Windows as well as on OSX. May you guys cant help me out?
So before I had my primary display plugged in on the HDMI output and my secondary VGA display on the DVI output with an DVI-VGA adapter. This combination worked in Windows but not on OSX because of the VGA.
So the main idea was finding an active adapter that for OS X the VGA display acts like an DVI one. So I changed ports to the following composition:
1.) HDMI-Output -> HDMI adapter to VGA -> VGA Display: that works perfectly, BIOS Screen is also here (but shouldn't or?)
2.) DVI-Output -> DVI-Cable -> Primary Full HD Display: doesn't work and I don't know why, it is just no recognised anymore
If I plug out the secondary display and reboot, Bios will come up on my Full HD via DVI, but on Boot of Windows neither OSX, the Screen goes black again.
Somebody knows how to solve this?
My guess its something with my 980TI, not a driver problem.