I would put the gas heater first and the electric second, and set both tanks to approx the same temperature. With this setup, the heating should be mostly done by the gas heater (cheaper). Electricity would only be used for the stand-by heating of the electric tank unless you draw more than 50 gallons at a time. I assume that you don’t want a second gas heater because of venting challenges?
Your plan would help a little bit, but I think that using both at full temperature would give you more bang for your dollar. Another idea would be to increase your thermostat setting if your existing water heater (and mix in more cold water at the tub).
(I’m an electrical engineer, not a plumber, so my logic could be wrong somewhere 🙂 )