1x PCI-E risers will only work on PCI-E slots. PCI slots use a diffrent voltage than PCI-E. If you connect a PCI-E riser to a PCI slot, bad things will happen. Not limited to smoke, sparks and fire. In a multi-psu setup it's important the PSU's share the same ground. The Add2PSU connector is an easy way to do that and it makes the PSU's come on and shutoff together. Using an ATX 24-pin splitter cable is another way.

https://www.amazon.com/Female-Power-Supply-Motherboard-Adapter/dp/B013HTWI9G

You also should only use the second PSU for the VGA power inputs. The main PSU connected to the motherboard should also power all the risers and accessories connected to the motherboard to avoid ground loops.



https://bitcointalk.org/index.php?topic=443540.msg4878774#msg4878774