One hot water heater runs out of hot water faster than the other.
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Use the plumbing analogy for electric circuits backwards.
The water source (12" (or 30 cm) city water main and the sewer are
the = and - poles of a battery. The 1.5" pipe is a small resistance
The 3/4" pipe is a larger resistance
the tanks and outlet pipes are equal resistances.
The two circuits are in parallel.
The lower resistance circuit will have higher current (greater water flow)
and so cool off faster.