Easiest way to do that require virtually no timing.

Cooper: climb up the vine to the platform between church and train track, he can then use showdown to gun down 2 target at once at once (church and train track)
Hector: barge into the room of the whiskey lady target, press showdown and queue in the axe attack.

Cooper can just jump straight down afterwards without getting caught
this way you only need to wait for the church guy to be in range (train track guy is always in range of Cooper at that location)