Although I do agree that the stronger rider should do longer pulls, the speed can vary if the weaker rider is able to recover and hang on while drafting. The weaker rider may only be able to pull at 20 mph and for a shorter time interval, then after moving behind the faster rider pulling at 21-22 mph, can easily maintain 22 without too much struggle. The stronger rider should pay close attention to be sure the weaker one doesn't get dropped and this should easily be communicated with only two people.