Harold Kestenholtz. I apologise for standing on my head too for far too long. Your methods of heating are very interesting, but the homeowner never stated from where he/she emanated. Could you be wrong?
You describe a preheating heater & a final heater, I dont know why a far superior professional would waste a heater in that way but I suppose that is where we Australians learn our mistakes.
From our perspective, wouldnt it be more responsible to supply two 75 gal hws & get 150 gallons of hot water than to supply two 75 gallon hws to get 75 gal of hot water.
We have some plumbers in Australia who do not know how to ballance a bank of Hws.