I actually got this done in mexico. Sombrero hats are easiest to shoot off. Basically just aim for the very tip of the hat. I tried doing this in dead-eye and it would be a headshot everytime. So instead of doing it that way, I did it manually in real time.
Hope that works for you. --- Former 360 Owner, PS 3 convert.
Shoot their hat off without killing them, and it doesn't have to be two people at the same time. Dead eye helps with this if you can't do it without them pulling a gun on you or running off. You can't mark them of course it it'll most likely be a fatal head shot so just use the slow mo of dead eye to nip their hats.