So I've not actually played Sekiro: Shadows Die Twice. It's not strictly because there's no co-op. Given that I've played all three Dark Souls games and Bloodborne, I suppose it's not not because there's no co-op either. Behold! Modders have come to my rescue. Here are three players attempting to fight through a section all playing as the One-Armed Wolf.
In a test video this week, "Loki," "LukeYui," and a third player all conduct what they're calling a "stress test." They say that the in-progress Sekiro Online mod has yet to ever handle three players simultaneously. It's still in early development, they say, but look! Three players. Nevermind the first few seconds where the game crashes. Nothing to see here, everything's fine.
After all three players are in and running about, proving their humanity by gleefully hopping around and squatting on top of whatever Sekiro calls its bonfires, they go on exploring the area together. Loki says that "while enemies aren't synced, they'll still register players as hostile." From what I can tell, it looks like enemies will attempt to attack the extra players but neither can damage one another. Or maybe each player is seeing the same enemies but in different locations. Hard to tell from the video.
There's a fair bit of stuttering, sure, as one player seems to repeatedly rubberband back and forth as they explore. In all though, it's neat to see progress towards strapping multiplayer to FromSoft's staunchly solo soulslike game. Good job modders, you're doing the good work again!