Co-op campaign!
... no crossplay.
... no local.
I hate this planet.
Evolution of Metal
Seeing how coop is implemented in this game, I can see why local coop just wouldn't work. It's not just that you have 2 separate characters to control, it's that you can have 2 separate characters doing COMPLETELY DIFFERENT things. One person can be taking on combat with their squad, while the other person is chasing down quests in town. That just wouldn't be possible in local coop. You could implement a lesser version of it local, but it would be missing a lot of the coop functionality of this game.
Xbox has the feature where you can use two controllers as one.
CoPilot? I'm not sure that could even help any game turn from online to local. The only way CoPilot expands coop mechanically is by turning single player games into local coop games, by sharing the controls. I did this successfully with Forza Horizon 3, where one person controlled the steering of the car and the other person handled the acceleration and braking. Surprisingly doable and quite fun once you got into it.
Ah, but I see you mean to allow 2 people to control 2 characters? or like half the squad? Sure, yea. CoPilot would allow you to do that without swapping controllers. Almost like Sony's phone swap thing they did with Man of Medan (right..? I honestly forget how that feature worked now).