If you "turn off" the second water heater, you will have to wait a very
long time for the hot water to reach the sink. You are better off
turning off the first, it seems, if this whole scheme is even plausible
As for a "solution" that is "cost effective": I can't help wonder why 4
people need so much water. Doesn't anyone care about conservation, the
environment, humility, thrift, and all those other things that would
lead one to just say "well, maybe our water consumption is excessive...
if we just used a normal amount of water, we could easily have 4 people
on one medium-sized water heater, and save some money, gas, water while
also benefiting the environment, our country, and the world."
No need to respond -- I know it's no use, and I shouldn't even post
such hopless rants.