Both are cooler master
550w 80+ and 850W 80+ series
That's not enough information. What are the exact model names?
And if they're only 80+ White rated, they're not Cooler Masters I'd consider using to power even one GTX 1080, let alone two; there are no Cooler Masters I'm aware of with that low of an efficiency rating that are actually decent PSUs.
I'd also never run a heavily used power supply either. It's the PC equivalent of reusing hypodermic needles.