Giant Revolt 2 vs Giant Anyroad 2 drag/roll resistance
If you're measuring on downhills, the Revolt will roll better because it weighs more, not despite it. While objects of different mass will free-fall at the same rate (all else being equal), the mechanics of coasting are different due to the effect of air resistance. A great explanation is available here.

There are better ways than roll to measure a bike's efficiency--like given the same energy input, same speed, and same wind profile (for the rider at least), which one goes farther on a flat surface? Or, given the same energy expenditure and the same flat distance, which one arrives first?

If you're concerned with efficiency, hunching over a bit more for aerodynamics will have much more of an effect than minor differences in rolling resistance, or slight differences in the weight of the bike.