I have made the switch to Linux after installing the drivers all the gpus are detected how ever the only the 6990 is working and the 6450 is not. it works on the drivers that come with ubuntu how ever not amd omega drivers.
system
fx 6300
6gb ram
amd hd 6990
amd hd 6450
This has traditionally been an issue with the Linux drivers for both AMD and Nvidia.
I can't claim to have had much luck myself, (as I don't have two cards from the same company), but you might be able to do something with a custom Xorg.conf.
i think someone posted that by running fglrx config -all worked
This is the thread https://forum.teksyndicate.com/t/revisiting-ati-problem-hd7990-ubuntu-gnome/77114/22
That was for a dual core GPU but it should be the same thing...
the Command is : sudo amdconfig --adapter=all --initial
this works to get the 6990 crossfire working but not the 6450