Cool and fun idea Felipe!
The only problem I got is I played alone controlling both cowboys and the right one always was winning when I pressed at the same time.
I think you could expand this a bit with. Maybe who will press first a sequence of a few letters. Any mistake will need a fix by pressing yet another key and repeating the whole sequence.
That would not make a game such "binary" as it is now, but maybe you want to keep it like this, it is just a quick idea. Maybe some targeting by pressing keys to direct the gun lower or higher too, things like that.
Keep up the good work!