Hello AnnasIphone,
It sounds as though there are really two questions here that I will address. You cannot have two separate users for one iCloud account. What you can do however, if they the two users want to continue using the same iCloud account, is adjust what information is being shared between devices. You can do this from Settings > iCloud. The following article explains what features can be turned on or off to prevent sharing.
iCloud: Change iCloud feature settings
http://support.apple.com/kb/PH2613?viewlocale=en_US
Sharing purchased music or applications from the iTunes Store or App Store would depend on what Apple ID is signed into the iTunes & App Store. You can have the same Apple ID signed in here if you would like access to all purchases made on this account or you can have a separate Apple ID. The following article explains how to sign in and out of the iTunes & App Store.
iOS: Sign in with a different Apple ID in the iTunes Store, App Store, and iBooks Store
http://support.apple.com/kb/ht1311
Thanks for reaching out,
-Joe