dual libraries sharing same music folder?

I am sharing the same laptop with my partner, and we each have an iphone. I understand how to create seperateiTunes libraries for us, but I wish to select a lot of songs she already has in her library. So upon creating my own, I scanned for media which then populated my library with her music...all fine... but then as I started to delete the stuff I didn't want, I checked that it wasnt being deleted from hers....it was being deleted from hers too. Pure nightmare.

So how do I get around this? I should mention my pc operates Windows 7.

Cheers