Thanks for getting in touch - this is quite a common area of queries and misconceptions.
The law as it stands (The Housing [NI] Order 2003 for anyone who's interested) states that a house of multiple occupancy (HMO) is one occupied by "more than two qualifying persons, being persons who are not all members of the same family".
It further goes on to define who is a 'qualifying person' and what it means by 'family'.
Qualifying persons are "persons whose only or principal residence is the house in multiple occupancy".
Based on the details of your letter, I would assume that's the case for your two couples, so we would look next at their legal definition of family insomuch as it's relevant to your two couples.
"For the purposes of this Order a person is a member of another's family if he is the spouse (or civil partner) of that person, or he and that person live together as husband and wife (or as if they were civil partners)"
I see that as being pretty clear that your two couples would be treated as four people from a total of two families. So, you have a house of multiple occupancy, but... do you need to register it as such? That's the question you're asking.
The good news is - you don't. The Housing Executive doesn't require you to register as an HMO if the house is "occupied by persons who comprise no more than two families."
One more person outside of those two couples and their families would tip the balance, so just be aware of that if they ask to move anyone else in.
I hope this was helpful for anyone else in a similar situation. HMOs are a topic that comes up quite a lot, so if you have any questions about houses of multiple occupancy - or indeed any other issues around buy-to-let investment - just drop me a line to firstname.lastname@example.org