Many ESCs today have a battery eliminator (BEC) built in to supply the 5 volts that the RX and servos require. In set-ups with more than one ESC, there would be more than one BEC supplying the regulated 5 volts. This 5 volts is always +/- a bit due to manufacturing tolerances, and the BECs can fight amongst themselves.
It is normal to disconnect the +ve wire of all but one of the ESCs at the servo plug, so that just one supplies the RX. The internal BECs supply the 5 volt requirements of their own ESCs.
Ghosts' point was that if anything happened to the ESC that was supplying the RX, the RX would stop operating, so using a dedicated RX battery and disconnecting all of the ESC + ve servo leads, whatever happens with any of the ESCs, the RX can operate the rest.
The idea of using the two batteries separately, one per ESC, has the advantage that a defect on one will not upset the other, given the proviso that if one is going to fail, Murphy's law dictates that it WILL be the one that is supplying the RX.