Hey, just chill out. As the other members have said above, as long as there is no misbehavior or complaints, you won't be able to take any action against both of them. Anyways, you have already mentioned that it's common there. Absolutely, it's so common in Western Culture - "seeing each other," which is not so common as far as Indian culture goes.
Even if you call them personally or talk to them, the message would not be conveyed in the right manner. It would only create confusion because Western people are so. For instance, remember the cricket controversy between Australia and India for using the word "monkey." So the same goes here, unless there's any witness of misbehavior at the workplace, you can't take any action. If you speak to them regarding this personally, there are chances that things might turn against you as you know "seeing each other" is so common there and for them, it's not an issue.
But one thing is surely under your control - "the rules and regulations." So just revisit the working hours policy or similar sorts of policies and try to bring out some connection. Send an email to all employees stating the working timings, and no employee should stay back for any reason. Staying back unnecessarily might prove lack of speed, efficiency, or other disadvantages in a positive manner in whatever way you know that will suit your workplace. Bcc it to your management also. You can also add that if anybody stays back at the workplace for any other reason other than work, then management has the right to question the same.
Mention that the email or notice is a "Gentle reminder" of all our firm's rules, regulations, and policies. The message would be conveyed in a methodical way now targeting the whole organization. If you still don't find any progress in that issue, then either you can take it to the notice of your seniors on grounds of misbehavior at the workplace or not following the rules, as you said it would just go on for a certain period of time and it would vanish after that. It's your choice from then.