I don't know how long you and your partner have been riding together. A couple of things could be coming into play. In your normal riding on singles, especially when not riding with your partner, you may be accustomed to riding faster than the two of you can go on the tandem. Your brain is still trying to go your normal speed so you are pushing harder that you realize on the tandem. The other thing might be riding style, especially cadence. On the tandem it is a compromise so you may not be at your optimum efficiency.