I'm absolutely sure about that.
Third and fourth passengers are "extras" -- can only be added if extra lifeboat space is still available for that muster station.
The first and second passengers are "givens" -- space is always available for them in the lifeboat.
(Assuming double or more occupancy, of course.)
I learned this when I started booking solo rooms, but wanted to keep the option available to be able to add someone down the road. I asked about booking a "John Doe" second passenger to preserve lifeboat space. My TA (who is also a friend of mine, and is also a top seller for Royal Caribbean for the last couple years) explained that even if I book the room as a solo, Royal will still allot two lifeboat spots for that room. The only time you have trouble adding someone is when you're trying to add a third or fourth passenger -- those are "space available" berths, whereas the first two berths (in a non-studio room) are "space guaranteed" berths.
Using round numbers to explain (not necessarily related to actual lifeboat capacity or room category breakdown) :
Imagine 100 rooms
50 hold only 2 people (100 total)
25 hold up to 3 people (50 double occupancy plus 25 third passenger = 75 total)
25 hold up to 4 people (50 double occupancy plus 50 3rd/4th passengers = 100 total)
-- grand total would be 275 passengers if all rooms are filled to capacity
But if the lifeboat only holds 250, Royal will allot 200 lifeboat spots for the first two people in each room automatically -- that way they are able to make sure they are able to put two people in every room. Then the remaining 50 spots get taken up by third and fourth passengers until all 50 are taken, at which time the remaining triple and quad occupancy rooms are limited to only booking two passengers each.
If they didn't allot lifeboat space to all the double-occupancy rooms before they are even booked, it's possible that all the triple and quad rooms would be fully booked (175 people) and then only 75 spots remain for the 50 double-occupancy rooms, which means that unless there are a LOT of solo cruisers in that particular muster station, several of those rooms would be completely empty for the sailing -- and empty rooms don't generate revenue.
Since third and fourth passengers are often children, the income generated is usually less for the third and fourth berths in a room -- drink packages, spa treatments, specialty dining, etc., bring in more money for adults than kids. So it's in Royal's best interest to make sure that they have the most first and second passengers possible, with 3rd/4th passengers being a bit less profitable, on average.
So that's why you hear stories of not being able to add a third or fourth passenger due to no available lifeboat space, but you don't hear stories of not being able to add a second person to a previously-booked-solo double-occupancy room. The first and second person's spots are permanently saved for them in the lifeboat before the reservations are even made.