First thought is what brand power supply are you using? If your using a cheapo non name brand and even some name brands that could explain your problem. On the back is the psu rated power amps per circuit. I suggest you check that against the power draw of 2 s1's.
Steve
Yep, cheap & non-branded PSU is not recommended for S1. I wasted 2 -non branded- 500W PSUs months ago, then I bought Corsair CX500. Now they run great.