One arm at a time for a focused bicep squeeze.
Keep your elbow still and rotate the wrist as you curl. Don't swing the weight up.
Curling one dumbbell at a time lets you pour full attention into each bicep, and the rotation from a neutral start to a palm up finish trains the bicep's twisting function, not just the bend. Each arm gets a breather while the other works, which usually means better reps deeper into the set. It is the thinking lifter's version of the basic curl.
The biceps do not just bend the elbow, they also turn the palm upward. Starting neutral and rotating as you lift trains both jobs in one rep, which a barbell can never offer.
Alternating gives each arm a short rest, so you can typically handle a bit more weight or grind out cleaner reps late in the set. Curling together is faster and more fatiguing. Neither is wrong, alternating is just easier to keep strict.
Slow down and pin your elbows to your sides. If your torso rocks or your elbow drifts forward to help the weight up, drop a dumbbell size. The bicep should be the only thing lifting.
