(powering 2 risers/cards with second psu..)
I think this was the issue.
How exactly did you hook up second PSU?
When using dual-PSU, you must be sure, that no current flows between both of them through the GPU.
For example powering riser with one PSU and the GPU with another through PCIe power.
Also, PSU grounds must be shorted.
And both PSUs must be on(or off) at same same time always. If not, one PSU will feed some power into the output of the another PSU.. and it will/may blow up.
Also, maybe you just overloaded PSU and it dropped +12v to.. say 10v or ripple have risen and killed the cards. Or both.