Originally Posted by big john

Theoretically, they should be faster everywhere. The power of 2 riders with the aero resistance of 1 (almost) and the bike weight can be a non factor. The problem with getting 2 great climbers on a tandem is they may prefer different cadences and have different ways of dishing out the power.
That's how my friends made it work, they had the same cadence. The captain was a good, not great climber on a single and the stoker was one of the best climbers I knew then. On the tandem they meshed so well they could drop great climbers on double centuries and other timed events.
They finished first on some local events.

The story of Joe Breeze and Otis Guy on a tandem was similar.