You don’t have to drain the tank to add the peroxide — just enough so that it doesn’t overflow when you put the peroxide in; make it three pints of drugstore peroxide, and only do the tank you’re going to use for now.
I’d turn down the thermostat on the tank not in use, but I don’t expect any pressure issues there. Think about it: pressure doesn’t set off the T&P when you’re not using hot water. The thermostat says when to fire up a heater to maintain temperature or recover after use, and when to turn it off. It’s only when something goes wrong that there is a problem.