If the couple are from the locality they might wish to just go home to sleep, otherwise it would be best to ensure that there are sufficient beds for all to sleep; especially if you will have a bit of a drive home the next day.
If you book a room for '2' then no the hotel won't want to think there are more people staying there. Some have a policy of no visitors in the room after 10pm so if you are meeting for drinks first you may encounter a barrier going back as a 4some to the room after that for play.
It's best to just ask the couple what they would want to happen, if they will want to sleep at that hotel then you could consider a family room. Or some hotels offer options of 2 bedroom suites. Personally when meeting as a 4-some I've experienced all scenarios, all bunking in one room, two individual rooms, or them going home in the early hours.
Just chat with the couple and you might find you don't need to stress the details; if they're more experienced they will have an idea what will work best...........