I stumbled across this the other day when trying to build a Mausoleum for a room build, I really wanted to have some separate rooms in that one room but you cant share it like that right?
Well there is a way around it.
As you can see from the picture below there are many areas inside my Mausoleum, but its all just one room! Hopefully its more noticeable in the second picture.
If you build a small room inside a bigger room when you click on the floor you will see that the game recognizes it as 2 separate rooms:(
Here is what you need to do to get around that.
1. Build your main room, then build the walls you want for the smaller room outside.
2. Pick up the walls from outside and place them where you want your smaller room to be like so:
3. Now when I click on the floor the games sees it as only one room
I tested it and it has complete privacy, so you can easily build a master bedroom with walk in closet and en suite and share it all as one room!
I hope this might come in handy for others and that it made sense lol