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.
Get stronger on pull-ups and you'll get better at just about everything. Here's how to do it, whether you can crank out a dozen or you're working on your first strict rep.