I have a house with 2 jetted tubs and 2 separate showers, plus another tub. One of the jetted tubs is enough to come close to draining the 50 gallon water heater that is currently installed. I can't fit a larger water heater up the pull-down stairs into the attic, so I was thinking of replacing the current water heater (old) with 2 new 50 gallon heaters.

There is a 1" gas supply line into the area, so that shouldn't be a problem.

My question is how to interconnect the two water heaters to get the best use out of them.

I could connect the output of the first heater to the input of the second heater and set the 2nd heater's temperature a bit lower than that of the first one. That way when the first heater starts to run low on hot water the second one would turn to add some heat. The problem as I see it with this setup is that the first heater would get much more use than the second one.

I could parallel the two water heaters, but getting the temperatures set the same would be quite a trick.

Instead of adding a second water heater I could add a storage tank with heat siphoning to circulate water from the tank back to the water heater to keep it hot, but it seems that a tank would probably cost more than a second water heater.

I can't split the hot water pipes to put part of the system onto each heater, the tees are all in the walls or finished floor.

Any ideas?