There is a scene in Special Request that might scratch that itch. Almost an inverse of your suggestion. Someone the woman knows sees her being molested while her face is hidden behind a curtain, and the man joins in. Not an NTR scene but I think it might still be in your kink domain here.
There is one that I think I found here. Someone help me out. If I remember right, an adventurer and her boyfriend adventurer stop in a town. One of the options is going to see a gynecologist and as you level up that thread, the boyfriend comes and is with her during sexual scenarios. RPG. As you proceed with a mission/thread/scenario you add hearts or checks so you can see how many more steps there are. I think that was one of my favorites but I suck with names and need to start keeping notes. I think this one is right on target for what you want.