I mostly use my desktop mac to manipulate my iphone 4s. Works fine. I'd like to duplicate that on my macbook pro. I connected the iPhone to the laptop and did a manual backup. Now I would like to sync the iPhone onto the two computers. But when I tried to sync the music files, I was warned that doing so would replace what is on the iPhone with my entire iTunes library. I don't want to do that. I got the same sort of warned with my iPhone apps. So, how do I sync the phone with two computers without destroying what is already on the iPhone? Thanks!