I have two old radeons which refuse to work as a second GPU. They are both of the same family. One x300, one x550.
I have a Radeon 290 which is my main GPU. Branded by ASUS, reference design. It is installed on a Gigabyte x99-UD4 board along with I7 5820K on Windows 10 64bit. It's great. However I occasionally need an analog output for a second screen, and 290 does not provide it. So i took x300, which does provide analog output, installed it and... got nothing. After some testing I got the following results:
- x300 works perfectly fine if installed in the slot closest to the CPU and is the only card in the system.
- x300 is invisible in the device manager if 290 is installed anywhere.
- 290 works from every slot
Testing the case further i took the x300 to my workshop and dropped it into my workstation - which is a dual Opteron 8435 on a tyan h2000 s3992e board and Windows 7 64bit. It has two pci-express slots, one of which is occupied by a Quadro 4000. I took that gpu out and installed the x300 and x550 togather. Intending to use their dvi-d outputs this time. Once again - I can get only one of those cards to show up in device manager.
TL:DR; - I can't get x550/x300 cards to work as a secondary screen.
What is the issue here? Are cards of that family incapable of being used as a secondary card?