my roomate and I share a computer how can we avoid having each others music and apps to each others phones without having to delete the stuff from itunes and readd it to itunes??

Can anyone help me figure out how to sync one iphone with itunes and sync another iphone with itunes without syncing the same items IE: music, apps, and other info. My roomate and I use the same computer and both have iphones, but every time she syncs her phone it puts all of my apps and music and info onto her phone. how can we avoid this happening without deleting the items from itunes and readding them when we want to sync our phones again???