Ah, okay. Even though I have the 1000w variant?I would advise against it. That PSU only provides 18A per 12v rail. The 290x can draw up to 25a, there's a high chance you'd blow your PSU and video card.
I was going to use a double 6 pin to 8 pin connector as well as a double sata to 6 pin connector, with both sata connectors being on different rails.
Would that negate this issue and make this safe to do?
Edit: I have decided to use a second ,dedicated PSU that I will be jumping before I boot the computer, thanks.