Heard an expert the other day on that topic. What sounds like a sane plan of combining the agenicies is fraught with problems. The main one is that the FBI has arrest authority and those arrests and the records required to do so are often the reason the agency is hesitant to share information. A leak of info after an arrest can get the entire case thrown out of court because of tainted jury pools, unwanted disclosures to defense attorneys, etc.

The CIA, on the other hand, tends to act more outside the legal system but it's equally as protective of their own secrets. In their case loss of life of agents and moles can result in someone possessing info that doesn't understand the delicacy of the source.

Mixing apples and oranges is a risky venture. Somehow the info from both agencies has to go into some central point containing experts knowing the fine balance of the two systems. That point has to be non-political and free to act and divulge info to the parties that have to act on it without worry of stepping on toes.