For a couple of reasons, it doesn't make sense to make these fights. #1, what happens to the title of the fighter who loses? Does he keep it? If yes, does it make sense to have a champ that just got beat by another guy at his weight? If, no...then what? A contender fight in that organization, but yet, neither contender actually beat the "champ" in that organization?
As "cool" as it would be, it just doesn't make good business sense and I don't see it happening.
Must happen.
#2 - Lets say the winner has both belts....then we have one title fight in each organization per year? That's also if the champion never has an injury. Contenders would sit out for year on a regular basis.
There won't be unification fights until they officially merge...just like the WEC, and that hasn't even happened yet.