Yup, you're exactly right. When both the LP and electric switches are on, the burner and electric heating elements work together to bring the water up to temperature quickly. No harm in using both at the same time for startup. Once the LP burner kicks off I switch over to all electric.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.