Correct, typically incoming water has a pipe pushing it at the bottom of the tank and the hot water flows from the top so it'll drain the hot before the cold. But running the heater or both heaters will help just not much. Might add 1 more gallon of hot during the shower might add 4 just depends.I don't think you can get a tank style water heater to stay warm even with both elements running. They have internal baffles to try and keep the incoming cold water from mixing with the remaining hot water. That must help. Once you have used what the tank holds the water will come out cold.
I will sometimes run the 1400watt element and the propane heater. It extends the hot water a little. But will never be able to heat the incoming 2 gallons per minute fast enough to act like a tankless heater.