We have a 2500 square foot house with 2 1/2 baths. House is 11 years old and the original gas hot water tanks are both 40 gallons. Both original tanks are plumbed in series. What are the advantages of having 2 hot water tanks over 1 that is maybe 80 gallons?
It's crucial that when you're using two hot water heaters, they are plumbed in parallel, and all the piping is symetrical. Otherwise, due to water naturally taking the path of least resistance, one of the heaters does all the heating, and the other is just a storage tank.
One big tank is likely a better scenario than two.