How to add music from a second computer to my iPhone 5

I just got an iPhone 5. I can manually control music from my personal laptop and put it on my iPhone. However I can't add it from my brother's lap top. When ever I try to click "manually manage" on his computer, it tells me to erase everything from my iPhone and sync his library. I did autherize hi computer. How do I add music from his computer with still keeping all of my current music?