During the Square Enix presentation for Marvel’s Avengers at E3 2019, it was said there will be gameplay with one to four players, but they left it at that. IGN followed up with the Crystal Dynamics developers and learned that the entire story campaign is single player only and has no multiplayer option.
After seeing the private Marvel’s Avengers gameplay demo at Square Enix’s E3 2019 booth, it was very clear the campaign was intended to be a single player experience. The demo was essentially the entire trailer shown to the public but they actually played out the full the gameplay sequences where you respond to the attack on the Golden Gate Bridge. As the level progresses, you’re put in control of each Avenger for a different segment and then it ends with a boss fight against Taskmaster. It was heavily scripted, kind of like Uncharted Tomb Raider, and had you switching from one Avenger to another for key story beats, so you can see why that wouldn’t work with two or more players.
Source: IGN.com You Can't Play Marvel's Avengers Campaign Multiplayer