I like the idea of the wife arranging to have a sleep-over with a friend and the husband knowingly taking a trip or going somewhere to do something fun

while this is happening so there’s a quantum-superposition type situation where nobodies hiding what is or could be happening but nobody asks and its arguably more sexualy compelling for both to maintain the “secret”

Nothing else need change, its many guys fantasy anyway much less being allowed to witness or participate in the proceedings