Chijoku no Uzu ~Nerawareta Futari no Niizuma~

Kouji's older brother suggests they play a "game".
The game is to switch wives, and see if anyone can tell the difference. So they switch wives, and have an altogether different kind of sex.
With this game as the incentive, both married couples spiral into corruption, which is is depicted in radical ways!

[Edited from DLsite English]