The heater that you have now is Gas or Elect?
How often are you in need of more hot water than you have?
2 water heaters, one as a pre heat could be one idea.
What is the incoming water temp?
If the first tank tank has a lower setting say lowest then it pre heats the water from 40F to 80F then the recovery of the second tank will be faster because it is going from the 80F to 120F...
Even the first tanks recovery would be better because it is only going from 40F to 80F..
A single tank would have more water to heat from say 40F to the 120F.
Does the need justify the added cost of tanks and fuel to heat the water?