Explain, will the psu not handle 2?
Don't know which 600 watt Corsair you have
One is a budget grade CWT with only 2 pcie connectors
The other is a bit higher higher quality CWT still only 2 pcie connectors
Don't know the full extent of your build but a pair of 570's could put a hurtin' on either of them that even I wouldn't contemplate, a higher quality 600w with 4 connector's, yeah I'd do it, but that's just me, others here would have an aneurysm