Well, you are in a conundrum. It seems you want the other guy, but even if you end things with your current BF, the other guy may not pursue you since he is the best friend of your BF.
It sounds like you are not that in love with your BF if you can become so obsessive about someone else. You have two choices. Stay with the current BF and try to get over this obsession with his BF, or end things with the BF and see what happens. Maybe his best friend won't care about his best friend and get together with you anyway. Maybe he won't. But at least you will be out of a relationship that you do not appear to be invested enough in to keep from craving some other guy.
Of course, maybe your BF will dump you and his best friend will feel free to pursue you since his friend is not likely to care as much. lol