Another good thing about using two batteries in parallel is that the equivalent internal impedance is cut in half, very beneficial for systems which draw gulps of high current (thing several digital servos in violent maneuvers) as there is less chance of getting a voltage depression on the receiver bus and causing a reboot on 2.4GHz systems. Another plus, it is often easier to find a place to mount two smaller packs than one large pack. Since switch failures are probably the most probable cause of in flight failure, the two switches also add to reliability. Note that most switch failures are where they start to have significant voltage drop due to contact contamination rather than a catastrophic open. That type of failure is very hard to analyze or even find when you are having intermittent drop outs or reboots.