it's only happened to me a couple of times in my life. for a while in seventh grade hanging out at my best friend's apartment after we found his sister's porn stash. we started out just watching them and after a few viewings admitted we were both rather turned on. we were pretty close at the time, so we felt comfortable enough to both whip out and stroke off to the porn. so it was never really a case of us jacking off for each other...more just we happened to be in the same room jacking off at the same time. though it was his reaction upon seeing my cock for the first time that provided one of the first hints that i might be on the larger side. lol