OK just to note after doing the math I'm back to one 65 gallon heater.
The new 40 gallon tank I'm looking at is rated at $277 a year average cost.
The 65 gallon tank is rated $320/yr.
Provided 65 gallons is enough, and I think it is..... as the two downstairs apartments are tiny as closets and have no baths just showers, 3 bathrooms total in the whole building. And let's not forget that the previous owner was renting all 3 apartments out on just the 40 gallon tank. For 20 years, apparently. Why they never complained, I don't know.
The 65 gallon tank would cost $207 more to buy at first...... but would save about $230 in heating costs every year over heating the two 40s.
( 2 x $277 ) - $320 = $230
That's assuming the old 40 gallon rates at $277 operating costs per year also, and its probably less efficient at that.