If you have more than one controller (I notice you used the plural) you should only use one of the BEC's otherwise the voltage regulators used in the BEC's will 'fight' one another.
Depending on your set-up, e.g. amount of servos and other equipment hooked off the receivers 5 volt buss, the BEC may or may not be able to supply sufficient current. Generally most BEC's supplied with Mtroniks ESC's are specced at between 1-1.5A. So that gives you about 5-7.5 watts.
An average sized servo tends to draw about 1-2 watts under normal load conditions, but they can easily draw upto 4-5 watts under heavy load or when stalled. Many digital or high speed or high torque servos can draw considerably more. Therefore if you have more than a couple of servos hooked up to an onboard BEC, either the servos need to be lightly loaded, or you may need to fit an external BEC capable of supplying more current.