On 14 mar, 16:10, Aaron FIsher wrote:
I bought a home a few months ago and didn't even think to look at the hot
water heaters.

A friend suggested today that I set the temperatures differentially; and
when I asked why, I realized I had no idea why or to what advantage two
hot water heaters would be set up in serial connections. (I'd have used
one for the teen's bathroom, and another, in parallel, for the rest of
the house.)

Anyway, what's the THEORY and RATIONALE for having two hot water heaters
in parallel?

Oops I missed the part of your questions below. I would say why would
you want them in serial.

Specifically:
- What's the main advantage (is is simply gallonage?)

Two 1.5KW elements in serial will split the voltage in two reducing
the power by four so you end up with two 375watt elements.

- What is the main benefit?

Two 1500 Kw heater

- What is the key drawbacks?

None

And, most importantly:
- Would you set the temperatures differently on them or the same?

I would reduce both to your convenience. You do not really need this
water to be so hot. If it is not to save energy, it is safer for young
children or old people if an accident would happen.

VD