Update, I don’t like mysteries, things not adding up so here’s a strawman analysis from public information.
Here is the battery pack 2inr19/66-2, I invite someone else to do searches and see if they find similar.
See it has one connector? Not two you cannot daisy-chain them (that I can see)
The Boom+ is like this - with the battery underside of the PCB (so could be two hidden there, there’s enough space for sure)
The side above of the PCB from above
and importantly from underneath side
Do you see what I see? There’s 1 connector for the battery
which goes to this off the battery
As such it is physically impossible for two such 6400mAh 7.2V cells to be connected. Or there has to be something else going on, say one of the batteries it’s plug removed and the wires placed into the connector of the other to be in parallel.
It does appear just from searching publically available information the specs of 13400mAh 7.2V is not capable of being true.
If I’m wrong, please do show how? A teardown and a photo would help figure it out.
This aligns also with the maths of power and charge times 5.5hr recharge times can at most in perfect situation with no headroom for trickle-charge 5V x 3A x 5.5h = 82.5Wh. 13.4Ah x 7.2V = 96.5Wh. So it is physically impossible to get that recharge time with the quoted 5V 3A to a quoted 13.4A 7.2V cell setup.
Happy to be corrected and challenged, it’s all laid bare above.