In my experience growing up as a twin, the statement "they did everything together, bathe, dress, go out drinking, live in the same room. Inseparable." is true. We were brothers but also best friends. We had a secret language known only to us. We often knew what the other one was thinking. We did double dates. We shared a girl a couple of times at the same time. We were each other's best man at weddings.
Nearly every jerk off was together for two reasons. First, because we were always together. Parents required anybody who invited us to a party or sleep over had to invite both. Second, we always seemed to get horny together. I knew he had a boner at the same time he did and vice versa.
Whenever we discovered a new jerk off technique, we taught each other which involved mutual masturbation.