I do know of and have spoken to a Dom who owns multiple subs.
He had lived with his first sub, for a few years, and incorportated another sub into thier relationship. She lives with them as well. He does own two other subs that live elsewhere.
He said that all the subs are treated equally, and no one is felt to be treated any differently than the others. Each one brings a unique thing to the relationship as a whole, and he ensures that everyone involved is compatible to the others as well.
He has come across subs, who have become jealous of the others, due to thier need for exclusive attention from him, and has let them go.
When he decided to take on more than one sub, he made sure the girls who wanted to try, were fully aware of what they were partaking in, and that they understood, what they were getting into.
When he plays with all of them, he allows one to be the dominate of the others, while he watches or plays with another one, and switches them up every once and a while.
He told me that you have to search long and hard to find girls who can be able to have this type of realtionship, and he has owned four girls now for about two years. He has been through a few girls, since some do leave because they find that they need something different.
He is still with his first two subs, but does not want to have anymore living with them, and prefers to own other subs, who do not wish to live with them.
He had considered it, but would rather do it this way.
I didn't ask how long the other subs stay, but I do know that they eventually leave, to find something else.