the load is shared across the batteries, as you get total power (watts) from voltage x current, and since the 2 batteries are in series, the voltage is doubled, so you need half as much current as you would need at the lower voltage.
So for example, to get 20 watts you can either use 10 amps at 2 volts, or use 5 amps at 4 volts.
Also since the batteries are in series you should keep the batteries used in the Sigeli as a matched pair and swap the cells every charge cycle, as the battery "in front" is getting the current from the battery behind it as well as its own passed through it, this would cause one battery to heat up a bit more than the other during use, causing one to age faster than the other. Hence why you should swap the batteries around every time you charge them.
In any case, I'd suggest anyone dealing with low ohm builds and high power stuff to read up on electrical theory, like ohms law and the like, the basics are pretty simple and will keep you safe.