Throughout the whole campaign of Red Dead 2, I never knew how to properly use 'dead eye' by this I mean, I would only use it for the shooting in slow motion. All I would like to know is how to use it properly, were you can pick how many shots go on each enemy instead of just one shot every time.
I think there's a tutorial in the campaign at the point where you unlock the ability to "paint" targets. I can't remember where that gets unlocked.
If memory serves, when the reticle is over the target, hit R1/RB to mark your shot. When you've finished marking all your targets, hit R2/RT to let off all the shots. Keep in mind that time is still passing, just slowly. If a target gets into cover while you're marking targets, you could miss your shot when you start firing.