Took a while, but finally, valid correct information showed up - from one "Tom Crump". To revive a rechargeable battery, you simply apply a voltage higher than the battery's. Even if both packs have been freshly charged, then connected in parallel, if one has a lower voltage, the other one will attempt to charge it to its voltage.
Some individual cells do fail open, but rarely. More often they short, resulting in a 3 cell pack, assuming you started out with four. You won't know it has happened until you see the smoke or it is "I ain't got it" time.
Others followed Tom with similar valid information, do yourself a favor and pay attention. Slightly off the subject, but "Propworn" also makes a lot of sense and I can vouch for that from personal experience. What personal experience? Well, first R/C flight in 1953 with home made radio, and, starting with Orbit Electronics 1n 1970, working with and using a lot of batteries till now. EloyM