If two coworkers is having a affair? I believe they should part ways and ust do their jobs,the way they should've.that way they can go on with their lives.
This is a very touchy situation. Frankly if these coworkers are not directly bothering you it isn't any of your business because if you do say something think of the problems it will cause for you at the work place.
However, if one of these individuals is your friend(s) then you have to be careful you would become the bearer of bad news and it could end up wrecking your relationship(s) let alone all the others involved.
Maybe an anonymous way of letting one of the mates involved know as to not getting yourself directly involved. Be careful.