You cannot do that. Each Apple ID needs to be tied to each separate account. Each of you must purchase your own copies of music and apps and sync to separate iTunes Libraries on the computer. The computer must have to user accounts set up - one for you and one for your fiance.
Sharing Stuff From Different iTunes Accounts (or Computers)
Here's how to do so using different computers:
Transfer what content you want to the separate iTunes library on each computer. To transfer content from your library to another library do the following. We'll assume this is between husband and wife separate accounts and different Apple IDs.
- Launch iTunes under your wife's login;
- Sign her out of iTunes, then you sign in;
- Select iTunes -> Preferences -> Devices -> Disable auto-sync when an iPod/iPhone is connected;
- Connect your phone, DO NOT SYNC;
- Select Store -> Authorize this Computer (only have to do this once);
- Select File -> Transfer Purchases (all of the purchased content on your phone will be transferred to your wife's library);
- When complete eject your phone, sign out of iTunes, sign your wife back in.
You can now sync whatever content you want to your wife's phone. Just remember all updates to apps obtained using your account must be done while signed in using your account: sign wife out, you sign in, update apps, you sign out, sign wife in, sync her phone.
This way you can share content (permitted under the EUSLA) and still maintain your separate logins. Do the same on your computer if you want to share content from your wife's library.
Just load whatever content you want to share on each others phones, transfer the content, then delete it from the respective phone if you don't want to keep it there.
Contributed by user wjosten and modified by me.