Depends.

Climbing, there are issues with inefficiency if the two have significantly different optimal climbing cadence (and therefore there is no 'perfect' gear). Even if they have similar but different optimal climbing cadence, one will be (typically) expending less than they would on a single, and the other is unlikely to make up for it.

On the level, or a gentle descent [gentle enough that they're still pedalling] the power used per unit speed is normally less, because the wind resistance per rider is lower [only a bit higher than the resistance of the captain alone, in most cases]. Estimates vary, but for reasonably matched [for cadence, not watts per kg] riders, something in the range of 5 to 10% seems normal.

This is assuming both riders are out for a workout when on singles and when on a tandem. I normally put out less energy riding alone on a single as I coast, and let my mind wander. On the tandem my wife doesn't like coasting except for butt breaks and when we top out on downhills. So it only happens at those times and as we approach intersections or tight curves. Very different dynamic.