There is only one champion, so I believe this fight must happen. It's a professional issue, and I believe they can fight. Deep down, everyone wants to know who is strongest. Friendship is friendship, but they're not amateurs. The eventual winner isn't my concern. What matters to me is how they win, and their strategy. This would be a mental battle.
http://content.usatoday.com/communities/mma/post/2010/01/machidas-dad-my-son-should-fight-anderson-silva/1