There's no problem if you use one battery, and there should also be no problem if you use two batteries and charge/discharge them both simultaneously (because then the voltages are matched).

The problem shows up when you try to "hot-swap" just one of the batteries and replace it with one at a different state of charge, as the README claims you can do.

What is the expectation here about functionality? They are using some kind of COTS battery system to keep the cost down, but at the consequence of this safety qustion. Should these people expect any buyer to antipate this? Its not really a consumer product after all.

I think there is no shortage of battery management ICs, but the number that can arbitrate between external power/battery A/Battery B certainly eliminates 97% on your Digikey parametrics.

IMO my expectation is that they not do that. If they want to offer hot-swappable batteries, they should either do it right (with a more functional battery management IC or a little homebrewed FET-switching circuit), or they should at least come up with a hack that doesn't threaten the battery safety. For example, have a physical switch to select between the two batteries and instruct the user to flip it when one gets low. Use a super capacitor to cover the short time while the switch is between connections.

For example, with this part:

https://www.analog.com/media/en/technical-documentation/data...

It feels to me that the fact that there are no readily available BMS chip that supports this type of circuit indicates something. But maybe I'm mistaken...