Oh wow. That does sound like a complicated situation. I don't have any insight into your marriages but I think the most important questions that needs to be answered first before revealing anything to your spouses, is: If you had to choose between your husbands and each other, who would it be? And if you are staying with your husbands primarily because you love them like you love each other or because of other obligations like children or finances.
If you'd rather be with each other and you're married more for financial and family reasons, then that makes things a little clearer because I feel that when all is revealed you might be forced into a situation where you will have to choose between marriage or being with each other.
If you don't want to choose between your husbands and each other then that has the potential to be even more complicated if you're forced into choosing in the end. But if your husbands can be open to having open marriages then there could be a chance for a happy beginning for you all. At least in your case, I think there's more hope since your husband seems open to the idea of you having relationships with other women, but I don't know with your girlfriend's husband.
Whatever the case may be, hopefully you all will figure everything out in the end. I wish you luck.