Just don't do it. You're planning a redundant hot water source that will only cause you grief. What's the worst that will happen if your new water heater goes tits up? A few cold showers. What's the worst that WILL happen when your old tank leaks or blows up? A flood, possibly no water at all, and sooner or later that old tank is going to have to come out. Pay a little now, or a lot later.
Up here home owners /business insurance won't cover any water damage caused by an electric tank over 10 years old.