I'm planning on buying a new R9 280X, to crossfire with my current one. However, I have a 600W PSU which may not be able to supply sufficient power for both, as each ~300W require under load, and the rest of the computer hasn't been counted in yet. Can I use another PSU and connect it's PCI-E power cables to the new GPU, and leave the other GPU and the rest of the computer connected to my current PSU? I have a MSI 990FXA-GD80 motherboard, capable of supporting both GPUs.
EDIT: Tested it with a GTX 580 using a 500W PSU for the motherboard, and connected a 350W PSU into the PCI-E, and it worked perfectly, but requires good timing.