Well in all cases a receiver battery failure would cause loss of control. Redundant receiver batteries would extremely reduce the changes of a complete failure. Many of us have found that quality LiFe batteries and especially the genuine A123 cells are extremely reliable though and coupled with quality switches, heavy power wiring to the switch and redundant connections to the receiver, the chances of a failure with a single battery is extremely rare. I would guess that most people use a single receiver battery up through and including 60 cc sized planes. Certainly nothing wrong with (2) batteries though and it does provide a comfort margin.