My Step-Brother is a year younger than me so growing up he and I went through puberty at about the same time. Back then he lived with his Mom and would occasionally come spend the weekends with his Dad and my Mom and me and my brothers. I can't remember how the subject of masturbation ever came up between us, but he and I were both avid masturbators at the time so whenever we'd get time alone (an older brother and younger brother were there also, not to mention our parents) he and I would never fail to end up jacking off in front of one another.
I'll never forget how hot it was to see him shooting his load, nor how good it felt to cum while he watched me. To this day I've never shot farther than back when he and I were together. We'd occasionally stroke each other a little but never to completion because of the AIDS hysteria at the time we thought if we came on each other we'd get AIDS. He and I spent a couple of years getting off together until one day we got caught by my younger brother, he freaked out and basically called us fags and from then on he and I were too embarrassed and traumatized to ever do it together again.