yup, sngle_chck hit it. as far as doing the exercises, you can do either both arms at the same time, or one at a time. try them both. I usually do mine both arms at once, and single arm once I start to tire, ot I am concentrating.
Also try doing the raises while lying on a 45 degree or so angle. lie facing the bench. I used to do those a lot after my shoulder surgery.