D & J,
I was in the mood for a western so I read your entire script. Actually took some notes.
Then I read John's comments;
In total agreement with John's advice and suggestions. No need for me to add anything here.
That being said:
I liked the "Magnificent 2" premise. AND some of the action scenes.
Some nice ending surprises (Calamity and Bea as Mary's mom) and the final brotherly duel.
In addition, a little romance, some tough ladies, some really mean, downright savage bad guys. Nicely done.
I think the slo mo battle at the end wouldn't be the writer's call, BUT it reminded me a little of "Peckinpah's Wild Bunch). Good stuff.
See if you can find a script from a western, read it and then
....Rewrite, rewrite, rewrite, you may have something here.