Family on my street have done this. Not a big family. Mum, Dad and 2 kids (now early 20's and mid teens but they have lived in the house all their lives).
Bought the house next door to them. The houses are standard 60's 3 bed semi detached houses with doors and hallways on the outside walls and party walls shared for living room, dining room, and the 2 double bedrooms.
They only use 1 of the front doors, but haven't fully joined the houses up. They have put doors between the dining rooms at the back and the 2nd bedrooms at the back.
Didn't want to pry too much so don't know about the small galley kitchens if 1 has been knocked through into the dining room, but I find the idea of the door in the 2nd bedrooms weird as that would be 1 of the kids bedrooms (3rd bedrooms are box rooms).