There are quite a few - search for
Swapping スワッピング
Swap スワップ
Couple カップル
The older stuff (early 5-10 years ago) was more believeable as being couples. Where as now they feel more like actresses/wannabes and not real couples.
There're a couple I think that are gaijin friendly. One called Opera has an English website.
Would be interested to know how you get on as my gf and I are talking about going.