Most Helpful Guy
LOL I can believe it threw you off. It is kind of surprising. But I find it understandable. Twin siblings really share a strong bond which is hard to understand for us twinless people. They have been together and super close all their lives. They probably feel "empty" when separated. If you want to have your boyfriend more to yourself, maybe you should help the other twin find someone to keep him busy :-)
Most Helpful Girl
I would suggest doing a Google search and some reading on the relationships between twins. It's not that unusual to share a bed. Sometimes the bond between twins is so strong, that it almost feels like a arm/leg is missing if the other twin is not near. Talk to him, and ask him to explain how he feels when his brother is not around, compared to when his brother is around. The answer may surprise you. Also look up psychological research/ journals/ studies on twins and relationships with girlfriends/wives.