[l23k4]

Neither of them could, there was a very slim set of circumstances where their goals might have sufficiently lined up for this to work. They studied it, arrived at the conclusion that it wasn't going to happen.

To frame this as something that should happen is just taking an overly simplistic view on how fighter jet development works. Even if everything went perfectly, it's far from obvious that a Franco-German designed fighter could be better than a purely French one.