PowerColor used a total of four 8-pin connectors on the Devil 13 Dual GPU R9 290X, which means the card can draw as much as 675W. The card ships with two 6-pin to 8-pin converters. Bear in mind that you need a 900W or 1000W PSU.
The reference R9 295X2 has just two 8-pin connectors and it draws 80W less than the Devil 13. This phenomenon can be explained in more ways than one. It might be down to circuit design, or it could the result of cherry picked chips – AMD could have picked low leakage parts for the R9 295X2. One advantage of getting a Devil 13 is that it you do not have to use a PSU capable of delivering 28A to each 8-pin auxiliary power connector. Most PSUs nowadays ship with a massive 12V rail anyway.