I researched tankless before I bought a new tank a couple years ago, but it would have required either
one new hole in the "skin" of the house in an inaccessible portion of wall under my deck, which is 30 inches
off the ground, because of the building code saying the vent had to be x number
of feet away from my furnace vent, or two new holes, one in the wall of the laundry room and another
in the outside skin through the garage wall.

It would have cost three times as much as a regular tank, and
I just didn't want any more holes in the outside walls. My water heater vents up and through the roof. Space
for it is no problem in my laundry room. Tankless would have been perfect for me, being the only one living
here, but installation requirements were the problem.

But never have I heard that one would need multiple units of tankless, unless someone is living in a mansion
or castle, in which case I suppose it would be possible.

N.