It's not surprising that Haunted Chocolatier looks like Stardew Valley since Eric Barone of ConcernedApe made both games. Stardew Valley has a great multiplayer mechanic, so it makes sense that ConcernedApe would add one to Haunted Chocolatier.
Will there be multiplayer or co-op in Haunted Chocolatier?
No, Haunted Chocolatier will not feature multiplayer of any kind. ConcernedApe's FAQ for the game states that no plans exist to make it multiplayer. It may be disappointing, and the answer early on may mean that there will never be multiplayer for the game.
As Eric Barone was making Stardew Valley, he played with the idea of a multiplayer option openly, and the publisher helped a lot with adding it. Chucklefish, the publisher of Stardew Valley during its release, no longer publishes games by Concerned Ape. Chucklefish has returned all publishing rights to ConcernedApe, meaning Stardew Valley is now self-published.
With Haunted Chocolatier lacking a publisher to add features that the developer may not want to add themselves, there is no way to make multiplayer if Eric Barone does not want to add it.