In researching the dual psu power issue i've heard some advocate powering the riser and PCI-E power connector on the card with the same PSU. Others say the psu that powers the motherboard must also power alll the risers. The slave psu should only power the PCI-E power connector on the card. Haven't found a definitive answer but it seems the people having cards die were using the slave psu to power both the risers and the PCI-E power connector on the card.
https://bitcointalk.org/index.php?topic=1761303.msg17827649#msg17827649In this case - one PSU is powering all the risers and one PSU powering PCIe power connectors - there may be current flow through the GPU from one PSU to another. In the case of one PSU is on and another is off - the one which is off will/may blow up because it will receive voltage on it's output. And the PSU which is on may/will blow up because who knows how many W the off PSU will consume using A through it's output ; )
And the GPU won't be happy at all. Prly also will burn.
However, ppl used to do it because of unpowered/un-cut risers - for example flat ribbon without +12v additional power. In that case riser -
because the risers voltage shares the line with the PCI-Express bus and if there is a tiny litte difference in voltage all hell break loose.
Nowadays, mostly ppl use USB risers, which provide all the voltages to GPU from it's external power connector. So they don't use PCIe bus voltages.