In my current relationship, I'm the only sub involved. But my first online Domme also had a real time sub as well. So I will take what for me is a historical look at the questions from the sub perspective.
1. Would you be more content if you were the only sub, or are you really happier being in a three-way relationship? If so, why?
For me it was neutral. The real time guy was there before, and after me. It made sense to me that she'd want something more than just online and she had it. I never felt jealous over it to be honest. In our case I still got a wealth out of the relationship, and as for her two male subs we were always kept seperate and each allowed the space to feel individually special, which in my case I can say I did.
2. When you have no contact with your dom for a while, do you get jealous? Do you think he is concentrating on the other sub?
In my case, perhaps very rarely. She was legitimately busy with work a lot of the time (travel, lots of corporate goings on and such) which could be a bit of a pain. That was more of a strain to me than the other guy was.
3. Would you ever consider issuing an ultimatum (“If I can’t be the only one, it has to end.”)?
In that relationship no because I would have lost that game for obvious reasons.. In my current one... I honesty feel so secure about the bond we have, that I doubt I'd ever feel the need if it came up.
4. What did you feel when the second (or third etc) sub was brought into the relationship? Or, if you were the second, how did you feel about the first sub?
We were always given our own seperate time, which in my view was wise. We didn't have the "other guy" brought up during our time together though we were all aware of what was going on. It allowed each of us to have time with the Domme without feeling left out, or neglected, or not having attention payed to us while the other one got attention.
My overall impression is this: It can be done, but it's obviously tricky business. One on one relationships have enough issues for people to deal with. Obviously more people = more issues. Is it for everyone? No. I can underrstand fully why some people are 100% against the idea of multiple D/s partners. I also think if a Dom(me) feels the right to multiple partners, you can't in seriousness demand your subs be exclusive to you, which as far as I'm concerned is a two way street Doms have to be prepared to deal with. No good leader is going to demand something of you they are not willing to demand of themselves.
Personally I doubt I would seek a "serious" emotional bond with more than one Domme or sub at a time, because that isn't what I want for myself. Nor do I personally feel that building a serious D/s bond with more than one person at a time is particularly wise. In terms of more "casual" fun (which to me means a relationship more about sexual play than deep emotional sharing), in my current one on one relationship, it's been made clear to me the door is open for that if I want it, but again I am not generally a person who seeks a lot of parntners so it's not a big thing to me that I am going to go looking for.
I think this is a topic that should probably be discussed at some point early in a D/s relationship if one is interested in the multiple partners in the future thing. Get it out there early, and make sure everyone is on the same page. That alone probably does the most to avoid hurt down the road.
I will say though: if one partner is for multiples, and one is against, I'm sorry to partner number 1, but the one who is against has to be the winner in the end or someone will get hurt.