Personal trainer Dakota Mitchell, replies: You should perform them both ways. Most people choose one version and do it over and over, but you can get much more benefit from including variations. Bilateral raises (using both arms) allow you to use a little bit more weight and balance your arms against one another. Unilateral raises (working one arm at a time) force you to concentrate more on the specific muscle you're training.
Bilateral
Pay attention to form - you often see this exercise performed badly because people use too much weight. Hold two dumb-bells in front of you with a slight bend in the elbows. Raise the weights until your palms face the floor with your arms extended perpendicular to your body. Emphasise the contraction in your medial delt heads at the peak of the movement, hold it for just a second, then lower the weights without losing muscle tension.
Unilateral
Perform one-arm lateral raises the same way you do the bilateral raises, only with one arm at a time instead of two. To stabilise your body, hold on to a post or an upright bench with the opposite hand. You can also perform one-arm raises on an incline bench that's angled almost vertically: press your opposite shoulder into the pad, maintain good posture and raise the dumb-bell until it's parallel to the floor. Use a slightly smaller range of motion than you would for upright lateral raises.