i honistly have no expericance with DFI but have you tried simply adding the crossfire bridge to the cards it might be the case that both cards need to have an active display where with a crossfire bridge only 1 card needs a display (if it proves to work with the crossfire bridge then either keep them crossfired or buy a DVI dummy plug as a "fake" display for the 2nd card)
also yes many miners use PCI-e x1 to x16 raisers for there mega rigs