Hi Tim. Sounds like a nice flying machine, hope you will post pics or video!
It sounds like the BMS on the second battery does not “wake up” because it’s not detecting current flow when you connect it (this is logical if both batteries are charged and have almost the same voltage).
One easy thing you could do is add a “start” button to the BMSs, from the manual it looks like this is possible, see below J5 “trig” connections.
Another way would be to leave the BMS out of the picture for discharging, and connect the ESC directly to the pack terminals - with the equal length cables as you said, and a fuse each. However to do that safely, you (or the controller) still has to take the cell conditions into account - cell voltage and temperature.
On this thread you can see the preparatory work for that, but it doesn’t exist yet and will need some additional wiring to make it work. It’s even trickier for two packs so maybe don’t hold your breath for this!