You can blow this easily, so listen up: have your ex over and start making out and getting busy with your gf. But make it VERY clear to your ex beforehand that she is to never, EVER reveal that you've been servicing her behind your gf's back.

If you do it this way, your gf will be really horny and pull your ex in while you're going at it with her. At the same time, your ex may just decide to join in because of your current arrangement with her.

Just beware that your gf doesn't know about these hookups with your ex. And NO gfs like secrets being kept from them, even if they're really open, horny whores.

If you do it right, you'll get some great threesome sex out of them. I just don't know for how long. Eventually your gf will find out you've been fucking your ex on the side and you'll get the boot. But that's not for sure, either.

Good luck!

That's what I was thinking Ray.

Some times things are better left as fantisies.
Why take a chance on blowing a good thing for a short period of fun??You could be left out in the cold alone!
Just my thoughts on the issue