People, please do not base your recommendations on rainbows and unicorns!
If you know the facts, give people advice.
If you are guessing, go somewhere else to get justification, don't do it by giving wrong advice here.
http://www.tomshardware.co.uk/nvidia-geforce-gtx-titan-x-gm200-maxwell,review-33151-5.html
"Torture – Full Load
While Nvidia reports the GeForce Titan X’s TDP at 250W, it never actually reaches this figure under normal circumstances. In fact, not one of the usual stress test applications pushes the card past 247W. With the above gaming loop average at around 224W, this typically leaves about 25W left for further overclocking. Sure, that doesn't sound like much, but we are working in the realm of hypothetical here."
600w is the absolute maximum you need for a single titan x system, it will even leave good headroom.
For SLI, add another 250w tops. You end up with 850 which will still leave PLENTY of headroom. The card WILL NOT struggle and you do NOT need 1000watts.
If you go SLI, do it now. The price of titans does not drop in retails, it never has and never will. At most by $100.
Trust me when I say you WON'T buy a second $850+ card when there will be MUCH stronger, cheaper cards around at that time.