As much as Mortarion and Typhus? If they could get those two in the same codex than I am sure Fabius and Fulgrim in the same codex. I imagine it will be more like how Mortarion leads a more Plague Marine focused army and Typhus a Poxwalker army, if that makes any sense.
I don't see Fabius joining back with the EC there is way to much hate there on both sides.
The thing is that Typhus is a (very) willing servant of Nurgle, and Mortarion seems aware of the fact that they're both puppets of the Plaguefather's will, so if Nurgle says they gotta work together, they do so. Same with Ahriman. He might have been exiled, but he was still loyal to his Legion and searching for a way to reverse the Rubric. The moment Magnus accepted him back he had no issue joining him because he never had an issue in the first place.
Bile on the other hand rejects the Chaos Gods, and serves no one but himself, while Fulgrim is far more gone than Mortarion or Magnus. Even Khârn is more likely to rejoin under Angron's banner, and he was the reason the World Eaters shattered into warbands.
Edit: I checked the (new) Codex, he doesn't have the Emperor's Children keyword there.
Edited by DeadFingers, 02 April 2019 - 09:13 PM.