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.
In Part One of my "no curls" arm specialization training series, I talked about the absurdity of most arm specialization routines. I proposed that most trainees do more than enough biceps curls and triceps press downs and don't really need more of either of them.