This is safe bot to me thats way below what this psu can deliver. There's 4 pcie slots and a max power rating of 750w/62.5A at 12v, so thats 187.5w from each pcie port safely with should be much higher. using 2 570s with undervolt from one output is not an issue based on corsair standards.
Except that it's not. For one thing the total capacity of the PSU has nothing to do with the maximum rating of the individual ports or the cables. RX 570's even undervolted, mining Equihash or dual mining can easily use 100-110W each from the VGA 8-pin. In the best case, that's 200W+ from one cable. Depending on the gauge of the wire used for the cable you would be far exceeding the the rated capacity of 18 AWG cable, which is 10A or 120W max at 12V. If 16 AWG cables are used, you would still be drawing over 200W from one PSU connector and it's NOT safe, especially for use in a mining rig under constant load 24/7.
http://www.rowand.net/shop/tech/wirecapacitychart.htm