If you're looking to add more available amp hours, what you propose will work fine. Just do a good job on matching the wiring, keep all connections good. I always use dielectric grease.
We lived off the grid for a bit over 3 years, using a generator to charge a bank of batteries.
During that time I added some amp hours by expanding the number of batteries.
The first thing I did when we got our trailer was add another battery to the ONE battery that came with the trailer, doubling my available amp hours. I would not hesitate to add a 3rd battery if I needed it, but the two we have will easily give us 3 days of dry camping without the generator, which is enough for us.
