@ RP I can do better than that. We met up with a Triad who have been together for 4years and living together for that long. Admittedly it was their second go at it. I actually got them to join this forum and he posted a post or two but they are more active locally. They are older (late thirties to early 50's) and children haven't been an issue. But this is the standard Bi-sexual female relationship with one guy.
@jlpanian There is a partial example of what you're talking about on here. Check out the Blog of a Mono wife in the lifestyle Blog section. They have two couples but it strikes me as a similar situation to what you're talking about.
Generally though if you aren't dealing with bi women then you're a guy with two "wives" which is polygamy. Or are you going to be happy if this other women has other intimate relationships?