Arthur Morgan and Red Harlow vs John Marston and Blondie.

The fight starts with a shootout between the two teams at John Marston's home. If one person on each team is dead, the fight switches to a duel for example if Arthur dies and John dies, Blondie and Harlow duel with revolvers only. The initial assault will begin with rifles and shotguns however in a 1v1 it switches to revolvers only. Who comes out on top?

Comes down to Harlow vs Blondie and I really don’t want to have to pick a winner

Interesting battle. For starters, John and Arthur, in character, wouldn't shoot to kill against each other, so I imagine they'd go after each other's partner. I believe Red Harlow defeats John Marston in a good fight: I simply think Red is the quicker shot based on winning multiple duels against three different shooters at once on two separate occasions, and beating Mr. Kelley in their duel who was stated to be a three time dueling champion.

I also think Red prevailed against the tougher opposition when you factor in the cartoony villains he beat (guys like Mr. Black, Colonel Daren, and Pig Josh). Blondie is quicker than Arthur, in my opinion, and a slightly better marksman based on the final scene in A Fistful of Dollars where he shoots down four guys while also disarming Ramón who already had his rifle out, and the scene in TGTBTU where he shoots the rope and four hats off in rapid succession.

Arthur won't go down easy, but I still see Blondie outgunning him. This means, the fight likely ends up settling into a 1v1 showdown: Red Harlow vs Blondie. Neither would miss in this scenario, so it's down to who gets the shot off first, and I think that would be Blondie more times than not.