My brother Mark and I (who I learned to masturbate from, by stumbling in on one of his sessions one day) lived together in the same room, and so we would masturbate in each others' presence fairly often.
We explored our orgasms in many ways. We had little games we would play. One of them was the stop and go, we'd masturbate frantically, and then at some point one of us would say 'stop' and at that point both of us would have to take hands off for 10 seconds. If you didn't take your hands off immediately, you were punished in some way such as having to take an extra household chore. It was such a rush to grab your dick again once the time was up. The idea was to call stop to keep yourself or the other from coming. This game was fun until Mark figured out how to get out of a chore by waiting until I was already shuddering in the throes of an orgasm to call stop.
We used stopwatches once to time the length of our orgasms. From the time of the first shot until the time the cum stopped coming out quickly. Mine was around 10 seconds every time. His varied between 7 and 10 seconds.
We used to try synchronized ejaculation. We would kneel, sitting on our heels, next to each other, and try to time it so our first strong splat would fire at exactly the same time. Mark usually came before me, but eventually he said he learned how to ride that fine edge just before the orgasm where one stroke can make you cum. It was interesting to say the least, his eyes would look a little glazed, his mouth hanging open, his hand moving in irregular, intermittent strokes on his cock, waiting for a signal from me to let go. His cock would twitch powerfully, precum would shoot forth from it, semen would sometimes shoot (but just 1 shot), and if I took too long he'd lose it eventually and all his sperm would come tumbling out. I knew he could ride that line for up to 10 seconds at best.
We would compete to see who could break tissues with their loads. One person would kneel and jack off into a tissue held from both sides by the other. You'd hold your penis steady when you came. We started out with a single 1 ply tissue each to catch the loads, you won if your load was too heavy (or shot too strong) for it. As we got older our loads got bigger so we had to upgrade the number of sheets but the same rules applied. The only problem with this game was sometimes if they managed to break the sheets, it would continue to fly and land on you. I had Mark's cum on my legs, in my pubes, and on my dick more than a few times.
We had a condom game. Each of us would have a condom and the goal of the game was to get it on, cum in it, and take it off. The winner was the first to get his filled condom off. After one time of this, we decided to drop the part about taking off the condom; it was too uncomfortable to rush to take it off especially with the head of the penis as sensitive as it was after shooting.
We had some other games, two brothers in their early teens have an extradordinary amount of free time. Maybe I'll share some more later, I'm done writing for now.