Do not use a splitter where you split the 8 pin PCIe connector into 2 - 8 Pin PCIe, you are asking for trouble that way. Even if you properly undervolt, what will happen if one day due to some software bug, it reverts the voltage back to stock and you end up pulling more current than before. Very dangerious.

There are only 2 ways to do with this. You need to get one of those heavy duty server PSUs and make sure you got enough 8pin PCIe connectors. Another method is to use 2 psu on the same rig. Buy that Add2PSU adapter and make sure you don't mix any voltages together to prevent current from flowing into one PSU to another and you should be good.

Another method is just get another motherboard and run half of them on one rig instead of all 6.