Is it possible that they can do that without creating two seperate libraries, that they just share the one?
I think it is possible.
On Itunes, in the top left-hand corner, click on "File". This now shows you many options. The Next thing you need to do is click on "Add File To Library". This then opens a box with all your music in.
If you have saved music from anywhere into Windows Media then they all should have saved here.
Click on any songs that want in your Itunes library and this will add them straight away. If the file you have selected is in a different format to what Itunes will accept, another box will appear asking if you would like to convert it to the correct one. Click "Yes".
All should be done!