I think that's right. Ming was just fine having her slave all to herself, but we got Henry so she'd have company when the slave wasn't home. They get along ok, but sometimes the way they "play" makes me wonder. Doesn't help that Henry is more than twice Ming's size and she doesn't have any fangs so he doesn't always get the message.I have a theory that though cats can tolerate each one under the same roof, they prefer to be the only cat in the house. If they've found another home that offers that and are happy with the new slaves (aka. adopters), they will happily abandon you.
Ming is in charge, though.