For maximum safety, when you have multiple PSU for 1 rig, you have to connect all motherboard connectors, HDD, some GC and ALL risers on first PSU.
Secondary PSU should only be use to power graphic cards. If you split risers between PSU, you still have a risk...
No. A GPU and its riser should be on the same PSU.
No, sorry, everything related to motherboard must share the same ground. By plugging risers on different PSU you have a chance to join 2 different ground and make your motherboard break down. In worst case it can start fire. Plugging only graphic cards on a second PSU won't make any problem.
I won't take the risk to see my house burn just for bad plug on a rig...
I agree. The PCI-E specification also clearly states the x16 PCI-E slot can be powered by a different PSU rail than the VGA 6/8-Pin aux power on the card. That's how I setup my triple and dual PSU rigs. The same PSU that powers the motherboard also powers all the risers. The other PSU's are only for the VGA power connectors on the cards. If there is more than one 6/8-PIN power connector on the card, you MUST make sure they are powered by the same PSU rail.
