The answer is probably yes as the inputs will be diode protected, but the Master will then operate on whichever BEC outputs the highest voltage, so say BEC1 gives 5,9v and BEC2 5.8v then the Master will use BEC1 until it's voltage drops to 5.8v which may only happen when the Lipo voltage on BEC1 is below the output voltage, so you risk running that Lipo to too low a voltage. Also the inbuilt voltage monitoring will be measuring the BEC output not the actual Lip[ voltage.
Probably better to go with 5 cell Nimh batteries or HV servos. If it's only one of two servos that can only accept 6v you can get servo inline BECs