On the Atwood model that I have you can run both. We do both if we are having multiple showers and need the hot water.
During normal conditions and when connected to a 50 amp service we always just use the electric unless its shower time and run both. I prefer to use the electric, since I am not having to use my propane when I can use the free electricity.
If its a 30 amp service, we run the gas since the A/C is most likely running.