Hiya Fate
As for poly, thanks but no. That is a whole bunch of drama I neither want nor need. A lot of that drama would be from me because I know me well enough to know that I would not be able to deal with the emotions and insecurities that would swirl around in me. It couldn't be anything but toxic. Maybe I'll "mature" lol but I doubt it and I truly don't care if I do.
As to your real question. The answer, to me, is threefold:
1) A dom can have as many subs as is workable within the relationships involved. A sub can have as many doms as is workable within the relationships involved. Is multiple doms tricky? Hell yeah, so is any group of people. How could it work? C-level business model? Good communication between the doms? The point is that there is no can and can't outside of what each is willing to accept without coercion and work, in good faith, toward.
2) Subs, stop putting up with it. If you do not feel comfortable with poly (or anything else) and it keeps coming up then step on it. You are equal in this - regardless the trappings - and if there is something that you know you are not up for then assert yourself in the context of the wider relationship. Time and place and all of that. There is no subbiest sub and if, God forbid, there is one then I don't want to meet her.
3) Doms are a problem. Not all of them, but a hell of a lot of them and they are fucking it up for the rest of us. Just like there is no subbiest sub there is no dommiest dom but damned if there doesn't seem to be a fierce competition for the non-existant crown. There is a check list that you must satisfy in order to be accepted into the brotherhood. I've not yet ticked off every box so I'm unsure how the induction ceremony works.
One of the required boxes is multiple subs*. This is because the more you can claim the more dommy you clearly are. I imagine that three is the minimum but I don't know if it goes by gross number or combined height or what exactly. A dom with only one sub is obviously not powerful enough to manage more than one sub - grunt scratch scratch grunt. The fact that not a single one of those lucky and select subs is getting near the attention that they require and deserve doesn't occur to these mongols because the only thing that matters is their duct-taped swollen egos. That's how they appear to the dommy committee. And if you are reading this and thinking, "He better not be talking about me?" Yeah, I fucking am.
A sub can't have multiple doms because the ego won't allow it. If you have two doms then it screams to the world that HE is not dom enough for you and you need a supplement. He may as well pin a picture of a two inch penis to his forehead and wear an "Inadequate" sign around his bowed neck (to those that are into that - good on ya! You do you
) He knows that he is inadequate, she likely knows - or she will know really damned quick - why broadcast it to the world?
So there is my far too long and uninformed answer to your very sensible and perfectly fair question. Sorry for the rantiness Fate.
Bliss
*Editted to add - I do not mean to suggest that all poly people fit this description. There are good and functional and fulfilling poly relationships and that kicks 9 flavours of ass. If you are one of those people and in one of those relationships then you are not who I am talking about at all.