well you could use your 6950 for that, should display to whatever is in PCI-E1, probably need put the 980 in pcie4 if its bricked. Anyhow, use just the PCI-E x16 1 slot and the second PCI slot, make sure PCI-E is set for primary display in the advanced northbridge settings. In the boot menu, PCI ROM priority, disable (not legacy, not sure what your choices are). Disable full screen logo. Also, you can use EFI shell to fix a bricked card.
In summary, to meet your goal you have 3 options:
use functional gcard in PCI-E 16 slot 1, with bricked card in slot 3 -> windows
use legacy card, select PCI, bricked card probably still best in PCI-e slot 3 -> shell efi or boot thumb drive with a flashing utility, just make sure to have your hardware addresses correct when you program the flasher
purchase another 980 ti and swap from primary and secondary in slots 1 and 2, assuming you brick the first one, so that the motherboard posts with pre-sli setup settings (second card not initialized but should be flashable).
Hate to be like, dont brick your card, but just about everything else is a better use of time: for the adventure of it though, see if you can find a PCI graphics card that existed post UEFI, those may not know how to be assigned addresses on current northbridge southbridge architectures (it may not take assignments other than primary or what it thinks should be secondary). I think a GT 610 or 710 comes in x1, check to make sure that's an option in your priority though.
let us know, and thanks!