http://www.parallelminer.com/product/18awg-pci-e-6pin-to-dual-8-pin-y-splitter-extension-cable-2x-62pin-cable-6in-each-side/
Safest way to power the risers.
It'd work just fine for GPU's that use 6-pins of course. But what do I know, I'm no expert.
Edit: Is it a good idea to power GPU's in two different rigs with the same server PSU?
With a server psu like this one it's actually designed to power two mining rigs worth of GPUs. This does require 240v in order to run it. http://www.parallelminer.com/product/2400-watt-power-supply-kit-for-gpu-mining-94-efficiency-200-240v-ethereum-eth-zec-dash/