it works for some and not for others like anything else.
its not a type of r/ship ive ever been in (unless you count cheating husbands lol) and it wouldnt wok for me, i dont have a jealous bone in my body in that area as far as im concerned my partners body is my partners body not mine and up to him to do as he pleases with it, though saying that id never do it unless told to.

my problem would be that im used to being the only partner so if someone else were to come along id automatically and probably subconciously be top of the heirachy lol and i think it would take a lot of time for both him and me to get used to the change,
id also have extreme difficulty if asked to Domme the sub like some subs are required to, i really dont think id be able to do that..in fact i know i wouldnt!

in some ways for selfish reasons i think i would like a 'sister' or a 'brother' it would be nice for me too,have an understanding friend,fun and company at times but for the reason i mentioned above i dont think sure it would work.

plus there'd be times id want to sit back and moan it's their turn to cook dinner, hoover or make the coffee lol

ive noticed a tendency that people have of putting themselves first and worrying about how it will work for them and forgetting that the new sub will need time and help to get used to the situation too if he/she's coming into an already etablished relationship.