Alternate, use them both.
Also if you don't want to injur yourself warm up your rotator cuffs.
The drawings aren't the best, but you'll get the idea.
Four Exercises to Strengthen the Muscles of Your Rotator Cuff
You don't have to do them all, but something to warm yourself up.
Before every chest workout and shoulder workout I do
#1 on that site, but standing up with both arms
and #2.