Your gf should want to want to do this or else it will be a disaster. Here are some scenarios:
* your gf is crazy about you, super secure in your attraction for her and she is open to bringing another girl in for sex, maybe a friend of hers she trusts and picks, probably not a girl you know that you’d pick.
* your gf is crazy about you, super secure in your attraction for her and is suddenly shocked you’re thinking about wanting an additional woman in order to feel sexually satisfied. Your gf is now insecure, questioning every moment the two of you ever spent together and is mad because she can’t enjoy sex with you the same without thinking of what you’ve brought up. She will always be thinking you crave for more than she can provide.
* You do as @pdworkin suggests, go off with two prostitutes, get this thing settled for yourself and keep your mouth shut about it otherwise your gf finds out, is absolutely mortified and feels the whole relationship has been a farce, she is tortured by her love for you but will never accept your love for her the same as before, it all goes to shit.
ask yourself, “how much do I love this girl? choose your actions accordingly.