Two tanks are simply for ease of construction and spreading out the water weight - there is no particular advantage in having two vs 1 tank. Some Rvs tee the two together and some have separate dump valves, mostly depending on location.
Yes, long dump handles can bend. This sometimes happens after a few years when the valve begins to stick a bit and you push harder on the handle to close it.
Not sure of the side differences. Are you talking about fiberglass vs metal or something else?