Hold one side in an isometric contraction while the other arm moves through a dynamic (concentric and eccentric) movement. You then switch sides.

This simple modification not only helps you to create a new training stimulus, it also creates muscular overload by increasing the time under tension (TUT). By increasing TUT, you increase metabolic stress, a key mechanisms for growth.

Related:  The 5-4-3-2-1 Shoulder Workout

Related:  Growth Factor Shoulder Training