"How'd [Sniper Greg] know Crooks was ready, right at that exact moment, to immediately start blasting 5 rapid shots? They weren't in radio contact I hope."

Have you ever seen the movie Sniper, starring Tom Berenger and Billy Zane? The movie comes out in the early 1990's when one could consider radio communication technology of the last 20 years science fiction then. Both actors portray snipers who communicate in the field by aiming their weapons at each other while looking through their scopes. One raises his head to intitiate communication while the other looks through his scope.

One can see the first Secret Service sniper team atop the red building nearest the shooter's position in some video footage clearly. Notice the sniper whose rifle sits atop the tripod. He appears to have "the drop" on Thomas Crooks well before the first volley of shots. The sniper raises his head and then responds by flinching to the incoming fire while pulling back his mounted rifle. The SS sniper provides the signal for the assassin to shoot Donald Trump knowingly or unknowingly; he looks for Crooks instead of Sniper Greg.