On a tandem each rider puts in the amount of effort that they choose. Most tandems have both people pedaling at the same rate (cadence), but each person chooses how much force to exert on each pedal stroke. Although you weren't deliberately 'soft pedaling' you were probably pedaling much softer than your DH.

Usually the captain is the stronger rider and tends to put in more effort than the stoker - so in that case the stoker finds the ride to be easier and the captain finds it to be harder than on a single bike. But I've ridden my tandem with stronger stokers as well and then the effect was reversed. And when the pair is evenly matched they may both find it a little easier than on singles due to the aerodynamic advantage - at least on moderate terrain.