"Stacked" isn't an electrical term, but if it came from putting two batteries into a tube, like in a flashlight, then it must mean they are in series, which does mean the total voltage is the sum of the seriesed batteries.
In other configurations, i.e. a flat pack or a box, they could be (but don't have to be) in parallel, which keeps the voltage the same but effectively doubles (if two and they are equal) the current.
Think about jump starting a car. If you do it right, the dead battery gets the 12 volts from the good one, and the starter spins. Do them wrong, and you put 24 volts into both cars. Not good, in that instance, but a good example of the difference between series and parallel battery connection.
Nat having taken apart either of the devices under discussion, I don't know which connection is used. But in either case, the electronics regulate the output voltage. I have seen pic of the MVP battery, and it seems they are in parallel connection (pictures weren't good enough on the wiring portion to be sure), so they would be using the 3.7 LI cell voltage, and doubling the current.
In other configurations, i.e. a flat pack or a box, they could be (but don't have to be) in parallel, which keeps the voltage the same but effectively doubles (if two and they are equal) the current.
Think about jump starting a car. If you do it right, the dead battery gets the 12 volts from the good one, and the starter spins. Do them wrong, and you put 24 volts into both cars. Not good, in that instance, but a good example of the difference between series and parallel battery connection.
Nat having taken apart either of the devices under discussion, I don't know which connection is used. But in either case, the electronics regulate the output voltage. I have seen pic of the MVP battery, and it seems they are in parallel connection (pictures weren't good enough on the wiring portion to be sure), so they would be using the 3.7 LI cell voltage, and doubling the current.