What the esteemed contributor to RCUniverse neglected to mention was that after a very short time, you had two partially discharged batteries. From what I recall of the posting (age, memory etc) it was implied, but not stated, that one of the batteries was not as good as it should have been. The internal resistance of the charging battery would resrict current. My experience with large lead acid batteries was that when one bank had been charged, it was a good idea to let it settle a bit before linking it back into the system, else the battery room got rather smelly with all of the little bubbles coming off the plates. Never blew a main feed fuse, though, so the current was probably within 0.5C.
The two batteries connected together can never be regarded as a "reserve". They will always, once things have settled, be at the same state of discharge (the glass will be half empty). Even if connected via diodes such that neither can receive a charge from the other, they will remain separate - the higher voltage one will supply all of the current until its voltage drops to that of the lower one, when that one joins in.
Just fit a larger battery, and/or get a warning circuit either by buying or building or get one of those moden radios that does telemetry and tells you to end your session.
Never plug a battery into a BEC output, it will not end well. The BEC is not designed as a charger, and the battery probably wants to settle at a different voltage.