How can I block facebook friends from each other?

[tkoji@ymail.com]

Let's just say my love-life has become complicated. My long time girlfriend is of course my friend on Facebook, but now my new girlfriend wants to add me. I want a way to add her, but block each of them from oneanother, so they have full access to my facebook like everyone else, but can't see each other. Someone must have a way to do this? Thanks!