Use one dumbbell to train both arms at the same time.
Two-Arm Overhead Dumbbell Extension
Stand comfortably with a slight bend in your knees, and hold an 8- to 15-pound dumbbell by the stem (place one hand on the stem, and the other hand on top of it), and lift it above your head so your arms are almost straight, with your upper arms next to your ears. Slowly bend your elbows and lower the weight behind you head, then straighten your arms back above your head.
Training Guide: Do 3 sets of 8 to 12 repetitions.