This isn't sitting well with me. It seems like you're almost trying to find a way to trick her into being a bed buddy. It's all well and good that your wife is on board with this but this other woman is a complete person too and not just a sex toy. If the two of you had met at a swingers event and both mutually just wanted no strings attached sex that would be one thing but here you are with a friend who you claim to really like and it seems to me that you are reducing her to nothing more than parts.
It's very hard to predict what kind of emotions are going to come out of having sex with someone. Even if you both decide ahead of time that you are not going to become romantically involved with each other sometimes these feelings happen. From what you said if either of you developed feelings then you would pull away. I can't imagine that would do much for her sense of self worth if she happened to fall in love with you.
If you want to peruse this relationship go for it but I would suggest talking to your wife about the possibility that it might become more than just a FWB scenario. Once feelings are involved things can get pretty messy pretty quickly. If all you want is no strings attached sex you might be better off at a swingers club. You can then continue to have the sexual tension friendship with this friend and just enjoy it for what it is. Getting involved with a friend either ends very well or very badly. Are you willing to lose her as a friend if things don't go well? Would that be worth the sex to you?
Everything will be ok in the end. If it's not ok it's not the end.