MLB: The Show 22 does have online co-op. Players can go online and play on the same team as their friends in 2-vs.-2 and 3-vs.-3 online matches.
Up to three players can play on the same team, and each gets a different role that can be changed even between innings. In 3 vs. 3 games, one player will control pitching, one player will control the infield, and one player will control the outfield. In 2 vs. 2 games, one player pitches, while the other controls all fielding. For batting, players switch off taking control of the batter and can edit the batting order, as well.
Quick matches and Diamond Dynasty mode are both compatible with online co-op. For quick matches, players will choose a team and jump into the action. Diamond Dynasty mode will combine each player's lineup to create one team. Players can adjust which players are selected to create the ultimate dream team.
Players will be able to send messages, emojis, and gameplans with their teammates during online play. Since this is the first time that online co-op is available in MLB: The Show, there may be some features or improvements that are released after launch.
For more MLB: The Show guides, check out Will MLB The Show 22 have motion controls on Switch? on Pro Game Guides.