There are so many what ifs in these situations that even with the best of intentions, i think it could go horribly wrong.
What if your husband finds someone and you don't? Are you happy to sit at home looking after the kids while he's out dating and having sex? Does he bring her back meet you?
What if this woman was everything you weren't, would you feel jealousy about the way he spoke to her, and about her, if he was very enthusiastic? What if he fell in love with her?
And it's ok for him to be seen around town, in the pub with her, infinite dinners with her, strolling hand in hand? By your friends? Your family?
Because it sounds to me like he wants the whole package of wife, companion and loving sex life with that chosen person. So I think this will turn out worse for you, because you are trying to engineer a person into a precise situation that you want, and I don't think you can do that with another human being who has different thoughts and needs sorry.
I also agree with the PP I thought these agreements were based on the extension of a good solid trustworthy and sexually active relationship.
Have you got someone in. Und for yourself already OP?