No the S1 was not overclocked when I tried, what do you mean by doesn't power both blades from the same 12V string ? Are you refering to using only wires that come directly from inside the PSU and are not chained to other wires ?
You PSU is listed as having 2x 12V rails. You need to ensure you pull power from the different rails. Maybe one for PCI-e, and maybe you need to get the other 12V from the mainboard connector ? Or some other connector.
A single rail PSU is easier. Use any connectors you want.