I believe bosch recommends this for difficult installs.........
of course it doubles all install costs, and supply issues like gas.
might as well use 2 larger units since their no standby losses size of first unit is meaningless....
but one could have 3 or 4 grand in the 2 tankless in series install.
in what lifetime would the cost be recouped?
a thousand bucks can pay for lots of insulation, better attic ventilation, some storm windows etc.....
fixating on hot water heating standby losses may leave other easier energy savers ignored.
i mean geez a home has so many standby losses, from cable boxes, tvs, etc etc............