This happened once
One time a friend an I were laying on the bed in the nude. He was on his back and I was playing with his cock. I was jacking him and he was just laying there. I wasn't jacking myself nor did he have his hand on my cock. As I jacked him off, I could feel myself getting closer to cumming. So, I jacked him like I like to jack myself. Pretty soon I could feel my orgasm building up and POW I started shooting all over him.
What was good about it, usually after I cum, my cockhead gets very sensitive and I have to stop. This time I didn't have to stop and man, it felt great. He finally came shortly after that. That was the only time that happened.