I will preface this by saying that I am not a plumber, just a handy guy interested in operating a home on as little energy as possible. If you only ever need 50 gallons of hot water at a time, I don't see why you would have to run both. I would make sure that the second one in line is the one you leave powered up. Otherwise you will get luke warm water forever. Does your home have a large Jacuzzi type tub that would empty a single hot water heater if you filled it up?