hi,remember itunes music download the price is for 1 download only. to make sure you not getting charge double. when you buy music on the devices, make sure you log out after used it.because both devices been log in, itunes dont know which one is which, then automatically download to both phone because of the same id account.
so make sure you log off the account when not used.
you can sign out by in your device go to "setting" --->> store ----->> sign out your account
hope that answer your question
please click accept. thank you
How do I set up my own account so this does