Who cares?

The reason for riding a tandem is because you have a partner that you want to ride with. The good thing is you always arrive at the lunch stop together so you always have somebody to eat with and you always have someone to talk with. Given that fact, whatever level of effort the two of you agree upon is the perfect speed.

If one of you feels a need to compare your performance with somebody else and the other one doesn't, one or both you may find riding a tandem not to be so enjoyable.

Whichever way your relationship is headed, a tandem will get you there faster.