I have a small Bosch 117000 btu and my water has been at 33f- 34f
incomming at -17f. Even at 33-34 incomming I never have to have the
Bosch on high to take a warm shower or it would be to hot. A 180000 btu
Rinnai will do maybe 3 water reduced showers at 34f incomming no
problem, and definatly 2.

An Electric would need 120A for 1 shower.

Why 2 tankless, One sized properly can handle alot of load. Rinnai is
the most efficient tankless unit I know of. Rinnai also has built in
freeze protection to -30f.