Iphone account issues

My wife and I both have Iphones and since I have had my iphone 4, when she tries to sync hers she it completely deletes everything from the phone, the only way to get the information back is to reset the handset and when she plugs the phone in it does not recognise anything she has on the phone. Its almost as if its a completely different account. My iphone on the other hand when I am uploading an app from itunes on the phone it asks me for her password and then mine there after. I have 2 questions really 1) What is the issue 2) how do I resolve this as I am presuming its an itunes issue.

Any help is greatfully received.

Thanks