Writing credits aren't going to be determined by contract the majority of the time, that's for actors and maybe producers and some other crew. At least for Hollywood productions like the show being talked about.

Writing and directing credits follow guidelines from their respective guilds and they can be very specific. And at times very contentious.

In summary: the credits OP is talking about are almost certainly the way they are due to union rules. Going by those rules it seems there were two distinct teams that worked on the scripts and Michael Tolkin was a part of both. The team listed first is in that spot because it is felt that more of their work appears in the final product than the other group. Or because it is felt both groups made equal contributions to the final product and then the credits simply go in chronological order. If there are disputes about this stuff it is handled by arbitration.