Hammer curls are an excellent exercise to hit the brachialis which is technically a distinct muscle from your bicep. It sits somewhere in between your bicep and tricep. However, when you develop your brachialis, your arm will look thicker. It will also contribute to your biceps appearing larger since the brachialis sort of pushes up against the bicep. Hammer curls train the brachialis as well as your forearms.
❌ It’s very common for people to do “cheat curls” on these where they use awful form and heavy weights. Hammer curls are an isolation exercise and you should control the weight to make sure you’re actually hitting the correct muscle.
✅ It’s not necessarily “wrong” to do each arm separately. It’s just less effective and more time consuming. Doing each arm simultaneously will reduce the rest time between reps for each arm so that your arms don’t get a break after each rep. You are also less likely to swing and use momentum when you do both arms together. You can bring the dumbbells straight up and down or you can also bring the dumbbells in towards your chest. Both are good and I’d suggest trying both and going with the variation you feel more.