Second (back up) battery on a Bafang mid motor

[Gionnirocket]

Verify your chargers are equal/close in voltage and charge them up. That's what I do. I only parallel my 36 V packs. I have four or five chargers that go from 41.7 to 41.9V on my voltmeter. In your case, full charge is 54.6V. It doesn't matter to me if one pack is 0.2 volt different. Not much current flows at those differentials. I've checked.

If you're one of these guys that change to 80% or 90%, you probably shouldn't parallel your packs. Reason is that at 80-90% SOC, the cells are still quite willing to take a lot of charge current. So any error on your part could result in high current. As they get close to 100% SOC, the natural tendency is for the cells to take limited current. so I believe I can be off a little bit and be OK.

I'm still carefull. I do check (with a meter) that my packs are close to full charge if I'm going to parallel them.

Yeah the desired SOC makes absolutely no difference... 80%, 90% or whatever ever you want is just fine.. It's the differential between the two batteries that is the important thing to keep an eye on. As long as the voltages are equal it is ok to parallel.
That said... I assume if you are spending extra time and money on this setup, you're looking for maximum distance and will be full charging.. But if you have an intelligent charger like the Satiator or my Solar Charger hack, you can easily do any SOC desired.