Dave,
I have seen instances where using two batts into separate regulators and then into the two power inputs on a Powersafe RX can cause problems. The Powersafe RX pulls power from the input with the highest voltage, and often two regulators will have enough difference in their outputs that the RX is drawing down one battery/regulator combination more than the other. After a few flights one battery is ready for charging and the other is 80% full.

As for the recent poster looking for an opinion on powerboxes with a powersafe RX, I would suggest leaving the powerbox out as in this case it probably doesn't give you anything that the powersafe rx and DX18 can't give you. Adding the powerbox will add complexity and introduce more points of failure for no benefit.

KennyMac