A good analogy would be the directing credits for The Menagerie - Marc Daniels was given credit for part 1, and Robert Butler, who directed The Cage, credit for part 2, even though Daniels had directed all the 'present day' scenes in part 2. Whether this was a DGA rule, or just a compromise worked out by Desilu/Roddenberry/whomever, I do not know. They certainly could not give 'co-directed' credit, as neither man was not on the set of each element of the story.
One thing is for sure, Daniels direction of the 'envelope' stands out as some of the best Trek direction EVER - especially the last few moments of the coda, with the whole sequence of 'Mendez' disappearing, the reaction shots of both Kirk and Spock when the Keeper explains the whole set up, Kirk's gentle 'dressing down' of Spock for not coming to him with the Pike 'problem', and then when Spock wheels Pike out, the very subtle way Nimoy lowers his head after he passes through the doors - a VERY emotional moment for Spock, indeed! And then the finish, with Kirk supposedly seeing the revitalized Pike and Vina, with the Keeper's send off message - again, Kirk's reaction is perfect - he has been literally beaten up emotionally throughout the whole incident - Spock apparently turning on him, stealing his ship, the whole trial procedure, losing his command - then the Keeper telling him that Pike has his fantasy life, and he has reality, and may he find his way as pleasant! Kirk gets to smile, straighten himself up, and regain his dignity - all subtly done and all done with ZERO showboating. Again, one of Shatner's best moments as Kirk. and all of this in less than five minutes!