I have played with overclocking, as have others and here are my numbers. The difference here is that I have also measured the current with an induction current probe. My current numbers are not that accurate but should be close enough. I get different values depending on whether I measure the positive or negative wires and which direction the probe is oriented.
As an electrical engineer, my conclusion is that one PCIe cable per blade should be sufficient, but the caveat to that statement is that I don't know anything about the design of the CCA and the interaction of the 2 plugs. I assume they both plug into the same power and ground planes on the board in which case just using 1 plug should be fine; although I hate to go against mfr's recommendations as they should know their product (although that may have had more to do with earlier versions which may have had problems that have since been fixed). I also doubt that 2.5 additional Amps from overclocking will hurt anything. The only other thing to say is that early on in this thread, people reported performance improvements depending on which plug they used. I don't know what to make of that.

218.75Mhz, 14.2A, 440Gh
225            14.6A, 446Gh
237.5         15.7A, 475Gh
250            16.7A, (My miner didn't like it, but I estimate 500Gh+)

(All amp numbers +/- 0.5A)

With 3 18Awg wire sets in a harness, 16.7A should be within spec but I also do not know the quality of the wires and cannot speculate about the quality of wires in your average power supply unit. This also assumes that all 3 wire sets have a good connections. There's also a caveat that I don't know if the weak point might be the connector or the traces on the CCA.

My conclusion is that to run 2 S3's at stock frequency, you'd want to make sure your power supply can deliver at least 700W to the 12V rail, and 800W if overclocking to 250Mhz.

My Antec Bronze-rated 850W supply consumes 730W running 2 S3's at stock (218.75) speed.

I hope this helps some people.