Well I'm a bit offended. Not at your questions Qmoq, don't get me wrong, I'll be answering those in a minute. But the "I think it's just greed" and "looking for a quick cum fix" statements are rather insulting to those of us in poly relationships don't cha think? Anyway on to the questions.

For the dom, I’d like to know:
1. Why do you think it’s working, when other similar relationships have failed?

I have to answer this with another question I think. Why do you continue to try for one on one relationships? I assume since you are still looking that you haven't had one succeed yet. I love my girls. What else do I need?

2. If one of your charges complained, what would you do?

Talk to her. Find the root of the problem and try to find a solution that works for everyone.

3. Why did you take on a second sub?

Because it was right.

4. Do you think either sub might not be totally truthful about having a rival for their affections, because they are worried that they might be the one who is dropped (or equally, because they like the other sub and don’t want to see them hurt)? This includes their response to this thread.

That's a loaded question Q. Everyone is afraid of being dropped, regardless of their relationship. People in one on one are afraid that the other might find someone better, people in poly are afraid of being dropped for their partners. It's common jealousy and the way to deal with it is to deal with it. Openly and without trying to manipulate the situation.

5. How would you respond to an ultimatum? Most good doms on this site
aren’t the “my way or the highway” kind of chap.

No I'm not the "my way or the highway" kind of guy, but lets face it; if your relationship has gotten to the ultimatum point it's over already anyway.

6. Do you ever feel as though you can’t spend enough time with each sub? If so, can you clarify question three?

Of course I do. Do you ever feel as though you can't spend enough time with a single partner? Work, children, life itself gets in the way. It's what you do with the time you do have that counts.

Maybe, because I have yet to find that special someone, maybe there is a little jealousy. (I know that you’re all suspecting me of thinking “Why does he have two wonderful people, when I’ve not found one?” and I’d be a liar if I didn’t say that I do think that at times.)
Like you I mean no offense when I say I think there may be more than a little jealousy here. Or maybe just some cynicism about relationships in general lurking about. Your questions seem to be more on the order of "Ah ha! I caught you!" then natural curiosity. I could be waaaay off base here because I'm a bit nettled at some of the other comments made so if I am I apologize and hope I've given you some good answers.