TDP is the power required to cool the component, not power it.
600W is cutting it really close, even on Haswell. I would look into a 700W unit, assuming you have an overclock on both GPUs, which would pull roughly 230W per on +12V, so around 20 Amps per, *2, that is 40 Amps at +12V for both GPUs, overestimating to 500W, with room for the CPU (80W on Haswell, ish), with some headroom for HDDs, ODDs, OCing, and such, that puts you right at 600W, which is not good for component longevity. 700W unit or 750W unit would be ideal.
Thanks for replying. Yeah was thinking I'd need a new PSU. I've been looking at the Corsair AX 760 or the 860 anyway. It wouldn't be for a while but is there any major difference between the 860i and 860? Also would it be better to get a single 780 or 780ti than get another 660ti and a PSU? Also if I were to get a 780 or 780ti then would I need more than a 600w PSU.