As i said above - it depends on how your batteries are set-up. If you stack them in series, one on top of the other, then you double the voltage, but the Amps remains the same.
If you run them in parallel the output voltage will remain the same but the Amps will double - and that is precisely the thinking behind battery packs, where a lot of batteries are wired together. The Tesla car is an example and so is your laptop.
Battery Information Table of Contents, Basic to Advanced
If you run an unregulated mod or a mech - the challenges are the same, because there are no electronics to save you, warn you of shorts and refuse to fire.
So the onus is on you and you alone to make sure that what you are doing is safe.
T