To pitch in with my opinion: while Mr. & Mrs. Smith definitely could work as a framework for Angel/Demon couples, I see that movie is much more fitting for a romantic relationship between a pair of Demon agents who are unaware that they work for separate agencies- it allows for more a more comedic and slightly more humanistic tone.

Also, I’m tickled pink that people found a renewed interest in this subject; it’s one of my more favorite topics to ponder on