Is it possible to have both my iTunes account and a friends iTunes account on my Air? I don't want to share anything between the two accounts, just want to be able to access both accounts from the same computer. Update separate iPhones, etc.
Sure. Just log in to your account and set up your iTunes library the way you want it. Then log out and have your friend log in to their account and set up their iTunes library.
Sure. Just log in to your account and set up your iTunes library the way you want it. Then log out and have your friend log in to their account and set up their iTunes library.
You can also create separate User's with each logging in to access their portion of the system. If you are just having your friend create an account for updating purposes you can actually just plug his phone / touch / ipad in while your logged in and it will update his product without x-fering your music / videos, etc and vis versa.