You need to create two login accounts on your PS3, one for you and one for your girlfriend (if you haven't already). Get your girlfriend to go in under her login, start the game and create a character then quit. Login under your name, start game and create a character, then your girlfriend can join in using the character she created earlier.
I don't think I explained that very well, but I'm at work and it's about six months since my brother and I did that!
In a change to my last plan, I'm going to capitalise on hype in the press on 'mummy-porn', but blend it with my love of tea*. My next book will therefore be called Fifty Shades of Lady Grey...
* Actually, I'm a coffee lover, but Fifty Shades of Monsooned Malabar doesn't quite work...