Question for those that know more about our batteries than I: is there a failure mode, such as an internal short, for a battery pack that would suck down the voltage of the second pack, preventing it from maintaining power to the Rx?

Basically, I think the suggestion of the Y-harness connection of a second pack is sound for redundancy (not capacity). This allows you to double up on the switch harness, which is a MUCH more frequent failure mode than a Y-harness. If you are concerned about the failure of a harness and don't have an empty channel connection on the Rx, connect the Y-harness through one of the existing channels. This way the failure of the harness will only kill one pack. Connect through one of the least critical channels, of course. That said, I have never heard of a Y-harness failing.

Bedford