If syncing specific music to specific devices is the only thing you’re after, then it would probably be better to have your iPod or iPhone sync with only a set playlist.
To get started, create three new playlists, one called “My iPhone”, and the others called “Her iPod” and “Her iPhone”. Then drag any music you would like to be copied to the specified device to the appropriate playlist.
Then plug in your iPhone, and open iTunes. Select your iPhone under the list of devices, then click on the ‘music’ tab at the top. Then select the radio button for only ‘selected playlists’, and then check the box for the playlist “My iPhone”. Then click on the ‘sync’ button in the bottom left of the screen.
Rinse and repeat for her iPod and iPhone, and the devices will only copy the music from their appropriate playlists. The other advantage is that you can then easily drag any music you want your wife to listen to (or vice-versa) onto this playlist, and it will be added onto the iPod/iPhone next time it syncs.
If you still feel you would rather have separate libraries (although this creates headaches when purchasing or adding music and so on), then I would strongly recommend that you simply create another user account on your computer through the control panel, as this at least makes things much easier and more logical. You don’t have to password protect accounts if you don’t want to.
Hope this helps!