From the standpoint of energy expenditure and leg tiredness I find a tandem to be a benefit regardless of the terrain. On the uphills it'll be slower than the faster rider on a single but still faster than the slower rider and on the flats and downhills it'll be faster than either rider by themselves.

The only caveat is that seat comfort can be an issue for the stoker due to not seeing upcoming bumps directly and reduced ability to make the little position adjustments we make while riding to relieve any discomfort. How significant that is depends on your wife's level of experience on the tandem. Has she done any long rides on the tandem? And how did she feel at the end of them?