how do you tell your partner "this play partner isnt working for me anymore"

weve been seeing the same couple for nearly 18 months and the wifes really clicked w the other husband. like really clicked. and im... ok with the other wife but the spark isnt there for me anymore n honestly it hasnt been for like 6 months. but i havent said anything cos my partners having such a good time and i dont wanna be the one to end it. now im starting to feel like a third wheel at our own meets and resenting going. how do i say "i need to step back from this dynamic" without taking away something shes really enjoying? do we close it down completely or do we find a way for her to keep that connection somehow...