I am 83 kg (183 lbs) and usually ride with my 54 kg wife. Yesterday I took an 80 kg male stoker from our cycling club for a spin on our Co-Mo Mocha. The ride was in a closed cycling loop in Buenos Aires with only cycling traffic (Circuito KDT).
We could move pretty well, 35-40 km/hr (22-25 MPH) cruising, 34 km/hr average in spite of some slow loops, maybe 2-4 MPH faster than with my wife. We rode alone most of the 40 km ride, some of the time we would get behind a group, but eventually we would get bored and pass them. Sometimes we were pulling riders who when asked to pull would not take the challenge. Only a fast pack of four riders would pass us plus the occasional triathlete.
However it takes getting used to a heavier stoker, bike handles quite differently, and the extra stoking power made it harder to stay smooth behind a pace line. Stopping and starting would work OK, but I had to be more on guard. Not quite sure of myself enough to try open roads with Buenos Aires car traffic with this new stoker.
Fun, but more fun with my wife, who lately has been finding lots of excuses to skip a ride. Maybe she would have less excuses if I took on another dainty lady rather than a fellow.