You are correct about both sails moving the boat more effectively than just one in light winds. As you do more sailing you will determine your own "iron jenny" rule, most people tend to fire up the engine if boat speed drops below 2- 3 knots or so. Drifting along on a sunny day with no wind, just waiting for a little breeze can be pretty nice though if you don't have time constraints.
SV Laurie Anne
1988 Brewer 40 Pilothouse