Having firsthand knowledge of having more than one submissive at one time, I can hopefully shed a little light on this subject.

One of the absolutely most important things when having more than one submissive is that they get along and that they are not jealous of one another. If one or both girls are jealous of the dominant's attention toward the other girl, then the relationship will probably never work.

Other things to consider are sexuality, experience, and areas of interest. If you plan to have a saexual relationship with both girls and expect them to have sexual relations with one another, then you should probably makes sure that both girls are interested in both sexes. If not, that's definitely going to put a strain on plans. Also, experience plays a key role in the relationship. If one submissive has been involved in BDSM longer than the other submissive(s), then she can help to explain what is expected and even help somewhat with the training of the other girl(s). Area of interest are also important, because one girl might be interested in completely different things than another girl. For instance, if you have one girl who only wishes to serve in a domestic capacity and another girl who only wishes to be used like a slut, then the two are unlikely to the same type of relationship with the dominant partner. This is not a bad thing. This just means that one girl is able to take care of the house and the service aspects and the other girl is able to take care of the sexual aspects.

In poly relationships, though, the two absolutely most important aspects in the relationship are trust and open communication. All partners must trust one another implicitly, and all partners must be able to communicate their desires, needs and concerns with all of the other partners. Withouth these two essential elements being met and encouraged, certain aspects of the relationship will begin to fester and the relationship will eventually fall apart.

One other thing that must be considered beforehand are outside commitments of each party. Are the jobs, education, family commitments, or domestic obligations of one or more of the parties involved going to prevent them from commiting fully to the relationship? If one or more parties cannot allot enough time and effort to the relationship, then they will likely become dissatisfied and need to leave in order to pursue their own goals of education, career, family, etc.

Polyamorous relationships can work, however. If the situation is right and if the conditions are all good, then a poly relationship can be very happy, very equitable, and very comfortable for everyone involved. I do agree, however, that there are certain limits to how much attention one dominant can provide to his submissive partners. I personally have never had more than two submissives in real life situations. I don't know anyone that's ever had more than that, but I would venture to guess that any more than about two or three submissives, and you're probably going to start encountering issues of insufficient time and attention. I would suggest that, unless the dominant is independantly wealthy, incredibly lucky at finding submissives that get along well together, and he has an incredible libido and an incredible amount of endurance, that palatial submissive estates would likely not be good ideas in real life. More likely, they would be the things of fantasy and fiction.