A train table is a good comparison and one that I have lots of experience with: There's a train table at our local library, and my son ADORES trains, and it's only a 2'x3' table with half a dozen traincars and not much track. We have been in every one of these situations at some point:
1. He and another kid start playing at the same time, and eventually the other kid is vrooming the traincar my kid wants or changing the track my kid laid out.
2. Another kid is playing when mine arrives and disrupts what the other kid was doing.
3. My kid is playing when another kid arrives and disrupts what my kid was doing.
In #1 I believe they have equal say in deciding what happens on the train table. I tell them that each of them will have to work around what the other one wants to do. The trains are Sharing Toys that belong to all of us. If my kid can't handle it and is flipping out, it's time to do something else and maybe return to the train table when nobody else is there.
In #2 I explain to my kid that the other kid has something set up already. If the other kid can talk, I ask him/her to show my kid what he can do here. If it's a toddler, I tell my kid that we'll have to be patient because this baby is just learning about sharing. If my kid insists on ignoring the other's plans and the other is getting upset, I say, "Let's go read a story. We'll come back to the trains later."
In #3 I tell my kid to explain politely instead of grabbing. Usually he needs help figuring out how the other kid can be involved: She can push this car; she can help you look for the track that will connect those two, etc. If my kid is too bossy and is hurting the other's feelings (or fingers!), I remind him that these are Sharing Toys and that he has his own trains at home; if he wants to play with these trains, he has to share them with other kids; now, can he work it out, or would he like to read a story?
Seems to me the same approach could apply to a dollhouse. Where there's another dollhouse available, I'd definitely try to interest the later-arriving child in that one. With a big dollhouse, maybe they could divide the rooms, dolls, and furnishings--much as "who's going to vroom the train" issues can be resolved by breaking it into two shorter trains.