From RAND:

In short, we find no evidence that historical joint aircraft programs have saved money. We also find that joint aircraft programs have obliged the services to accept unwelcome design compromises, have contributed to the shrinking of the military aircraft industrial base, and might have heightened the strategic and operational risks for the services and their pilots. Consequently, unless the participating services have identical and stable requirements, the Defense Department should avoid taking a joint approach to acquiring future fighter and other complex aircraft.