Mulitple iTunes accounts on Mac?

Basically, myself and my partner have our own laptops with our own iTunes (music/apps etc...)

We are thinking about getting rid of our laptops and buying a Mac Mini to share but is there anyway that we can create two iTunes so I have my library organised with my music and apps and she has hers?

Also, if this can be done...currently when I purchase a song it is organised and the file is saved within my library in 'My Documents' if I do this when we are sharing will it still do the same and file in my Library or will the files for our two libary be shared?

Thanks