I have seen the tandem system used only once, by a co-worker. He had a big unit, working on night-current only(very cheap in our country), and a small unit after it, using day- and night-current. The small unit would quickly heat(big heating element), and would be fed with cooler water later in the day. That combi had a rather constant water temperature for a reasonable price. Thermostats were the same on both machines.
Long time since you were a teenager, huh? A long hot shower can feel real friendly sometimes.
The water is pumped out of the ground from a well and then kept in two five thousand gallon tanks. So, the cold water is probably just a bit colder than outside air temperature. I never noticed it being particularly cold (it never freezes here) but I never thought about it before.
WH's in series are a standard for geothermal systems. The 1st WH gets the pump circulation from the geo unit and recovers the heat to raise the water temp. The temp is increased by about 25-40 degrees and the water goes to the 2nd WH where it is heated by elec, gas, propane, or solar to the final temp. There is no other heating elements energized on heater #1. My system utilizes this and has been quite effective and economical for 16 years.
I fail to see how too in parallel are going to allow for a higher GPM. The GPM is going to be largely determined by the incoming pipe size and water pressure. After the water leaves the water heaters, it flow through the same pipes, so again the flow rate is going to be determined by those pipes, faucets, etc. Following that logic, I could just cut out a small section of 1" pipe in a water system, replace it with too parallel sections, and voila I'd have a higher flow rate. This would only be true if the eqpt being paralleled offered signficant flow restriction. Not the case with a water heater.
Not without replumbing the house he bought, which didn't seem to be the objective.
That's a new one and quite unusual. Whover set this up must be a survivalist.
Fot the use it is set up for I would set the first (tempering) tank lower than the second tank, or as mentioned previously, shut it off.
Having it TOO low just encourages bacterial growth, particularly if the hot water demand is low.
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