Most ESC's unless they are high voltage ones have an internal BEC. If the ESC's have BEC's you need to disable both BEC's by removing the red wires if you are using a separate battery for the receiver. If your not sure if the ESC's have BEC's, power up one motor and don't plug in your receiver battery. If receiver powers up ESC has a BEC. Sorry to be a PITA but I have witnessed this same issue before and it was cured by eliminating one or both of the red wires.