SylvanLMP
I happen to like piping my hot water heaters (2 or more) in parallel with all piping equal for proper draw with a heat sink of 30″ (28″ according to the book) to prevent stratification of hot water into the colder line (Supply).
Remember heat goes to cold so during non usage time the hotter water will transfer
(natural cavatition) to the colder tank and you will have transfer between these two tanks VERY inefficient.
Here your preheating water to go from one tank to the next with lots of stand by losses and ILL bet you a donut you didn’t put in the loop for a heat sink ( NEVER A CHECK VALVE)
If one tank fails in Series your system is DOWN END OF GAME , in a properly piped parallel situation you can isolate one tank via valves.
No preheating is needed both tanks are set for the desired temperature as they draw equally.
I happen to like my return piping piped into the cold water supply to temper the water entering the tank, less possibility of thermo shock, BUT Hey that’s my hang up from being a certified boiler and pressure vessel inspector.
To fine tune your system you can add (3) a few gauges to check the outlet temperature of both heaters then the actual temperature going into the twined in piping (header)
Have fun.
