It's cheaper to get the Corsair CX500 (compared to a Corsair 750W or 850W for two S1s) to power each S1 at stock clock which draws up to 390WAC (usually around 385WAC) at the wall if equipped with a second high-volume "pull" fan. At 85% PSU efficiency, the S1 demands up to 332WDC (390WAC*85%) from the CX500 which results in a 73% loading (332WDC/456WDC) on the PSU's +12V rail. It seems there's some headroom if one is inclined to OC the S1 and push the CX500 a bit (I have no idea how much since I haven't attempted overclocking and don't plan to). Having a dedicated PSU for each S1 also affords flexibility and the ability to avoid having more than one S1 to go offline when a single PSU ceases to function. Best of all, ATX PSUs are PNP (no soldering needed).
The CX500 can be had for even less since the Egg always have $20 rebate offers and instant discounts (promo codes) for them quite regularly which could bring the actual cost to at least half. Since I've been purchasing these PSUs since last year and have acquired a few $20 rebates from Corsair in the form of $20 Visa prepaid cards, I've been able to get them at no cost at all. The rebate cards have to be converted to Egg gift cards and apply them to the purchase. Currently, there's no rebate offer on the CX500 at the Egg but the CX600 (with additional $12 off w/ promo code), CX500M (Modular) and the CX600M (Modular) do have it. Here's a comparison chart:
Edit: The "M" (Modular) version of the CX500 may not be ideal for the S1 as it has only one PCIe modular cable that cascades into two PCIe connectors that seem not long enough to cover the distance between the PCIe receptacles on each S1 blade (I'm not sure since I haven't physically tried them on).