I've tried it, and it just didn't worked at all. I could boot into windows, but as soon as the graphic driver was loaded, the system would lock, even in safe mode. The same card in the same motherboard slot without the cable worked just right. My cables were pci-e 1.0, but it should not be the problem, as pci-e is backward compatible if i remember correctly.
Then i bought a motherboard which can fit four double slot cards.
Problem solved and a 950w power supply gone (a week after installing the new mobo and all gpus) in smoke
Maybe 3 gtx 460 and a gtx 275 co-op (5 gpus total) with a heavily oced i7 970 was too much for him...