She can -
a) Hold milk bottle with left hand ; Play with belly button with right hand
b) Play with belly button with left hand, Hold milk bottle with right hand.
No problem with switching at all. As long as she has her bottle and her belly button, she's a happy kid :)
psss: I wonder how long this milk-belly button routine will go on. She'll blissfully lift up her dress and expose her panties to reach her belly button !!