We run all of ours at 393. Exact power with an 80Plus Gold Rosewill 750M is 410W on average. That puts it at about 365W draw from the PSU, which is 30.4A on the 12V rail.
For two Ants, you'd need at LEAST 60A on the 12V rail. I'd say 65-70A would be ideal. That RM850 gives off 70A, so you should be fine with plenty of headroom.
I have a corsair 850 that cant handle two overclocked S1s (393MHz).
What model PSU do you have? If it's not the RM850, can you verify the 12V Amp draw is the same 70A? If it is the RM850, you may have a bad unit.
This bitch: CORSAIR HX Series HX850.
But may be something related to the line, I'm not sure.
I have 3 S1's and one Cube.
I have another Corsair 850PSU (I believe it's a TX).
Currently my setup is the following:
TX850W: One Ant, Half Ant, Half Cube
HX850W: One Ant, Half Ant, Half Cube
If I put two Ants in the HX850W the PSU wont start.
The PSUs are connected to two different sockets, those sockets terminate in a single 20A switch.
I`ve got a HX850 supplying two ants at 400mhz. Pulling 860-870w from the wall.