From my experience the microwave takes longer to heat two bowls of soup than to heat just one.
But it's not so obvious to me why this should be the case.
Each bowl of soup is a single thermally isolated system, and the same power radiates across each one, so it should take the same amount of time to heat both, no?
Edit: I'm not convinced by the below answers. The answer should depend on the specific mechanism by which a microwave works, because if we ask the same question for a stove instead of a microwave, I would guess that two soups placed on a stove top would take the same amount of time as one soup, as long as the soup bowls have the same geometry and thermal conductivity.
Best Answer
The interior of a microwave oven functions as sort of resonant cavity, with the food items absorbing some of the microwave energy. So the more food is loaded into the oven, the lower the effective Q value of the resonant cavity, and the lower the equilibrium intensity of the electromagnetic waves inside the microwave. This means that the food absorbs less energy per second.
I seem to recall that, as a rule of thumb, tiny quantities of food (say less than half a cup of water) take a roughly constant time to heat. Large quantities (more than a quart of water) scale almost linearly, because they are absorbing almost all of the power output by the microwave.