As far as I am aware, there is no such phrase that exists in English. Usually (speaking from an american, cultural perspective), couples who are no longer happy with one another, live separately, and have other partners would be considered to be in a bad marital situation. Perhaps if the family were to see the couple like that, they would urge them to get a divorce rather than continue to live in each others presence unhappily.
To express the idea of "show-window couple," you would simply need to describe it in English. You might say "They are only married for show" or "They are only staying married to save face in public." However, I have never heard a particular key word that would describe such a situation.
I hope this helps! If you have any other questions, please let me know.