shouldn't it be enough to create an xorg.conf mentioning the correct pciid?
I tried to do that, and it didn't seem to work.
The primary display is the 6870 (I can't physically swap the cards due to the motherboard manufacturer putting the SATA connectors at the end of the PCIe slot - as the 4770 is slightly shorter it fits in that slot without snapping the cables off).
I'll try hiding the card as suggested above , otherwise I'll stick with the power-lead trick (works perfectly well and I assume it won't break anything).
Yeah, BusID "PCI:<slot id>" work on here to select my r600g driver( using r600 + 6850). But now I use fglrx 10.11 on my 6850 until open sourced driver comes. It's quite stable and fast if I don't start GIMP (or X kill itself). You can also remove "unsupported logo" from driver within 2 line bash hack.