Stock fans are power hungry, I don't want to try split power for two fans from one header. I take only pwm-signal from header, feed it to two fans and they have their own pci-e power-connector. And speed signal returns from fan1. Common grounds with all headers ofcourse. It's easily done with a couple of smd resistors and caps + 1 diode and electrolytic condensator. No fear to burn controller (this regards S5) or header. Cable that feeds power to the controller can't handle much current. I copied original circuit from S3 (same circuit is used in S5) and modified it a little bit.