I've combined powersupplies before with my GPU rigs and my recommendation would be not to risk it. I can only speak from personal experience, but they are a mixed bag of results. Sometimes they worked perfectly fine. Other times when switching on the second PSU connected to the motherboard, the bios would reset and cause random issues. Still other times, the GPUs connected to a different PSU would behave differently than the GPUs connected to the main PSU (eg: fans 100% permenantly). I'd say don't skimp on the powersupply with expensive hardware.