It will all depend on the compatibility of your cadences and riding styles... and how well each you are at making any needed compromises to find a happy median for differences.

Some evenly matched, strong teams are monsters on a tandem. Others just can't work out the compromises needed. It's a very individual thing.

Your performance on the tandem will be a by product of your efficiency. In general, most sport / recreational rider tandem team area bit faster on flats than either of the individuals are on a solo bike. Hills are where the differences in compatibility come in to play. There are some tandem teams who simply fly up hills, while others struggle. In most cases, a strong single bike rider will find that they are slower on climbs on the tandem even when they climb well on the tandem.