Thank you so much for agreeing to answer questions, Viridian! Right now, I'm planning on having it be two friends sharing one partner. I guess my biggest problem is that I just don't know how these types of relationships begin. If you don't mind me asking, how did yours start? Also, if it's possible, where would I be able to find your stories? I've tried finding stories about this subject, but I haven't found any yet.
I'm afraid you won't find my poly book out on the market yet. It's on submission right now. Thanks, though.
I'm not sure how much my personal story will help, but sure, if you'd like.
My partner and I started off as monogamous. We're pervs, and we never really saw anything wrong with sleeping with other people. It was a new concept, though, so we agreed to be careful with it. We agreed we would ask each other permission beforehand and talk about it, and we would stop if it became weird. So at first, we weren't polyamorous (allowed to have relationships with other people) we were in an open relationship (allowed to have sex with other people).
Neither of us were really into casual sex, though, so there wasn't much point in being in an open relationship. After a while, we talked about it again. We both agreed that it was okay to be in relationships with multiple people at once. Eventually, he met his girlfriend. They hit it off immediately, and he asked if it was okay to date her. I said sure, and now they're dating.
Not very dramatic, I know. The gist of it is, we were both fine with polyamory. Neither of us ever actively looked for new partners. He just sort of found one, and we decided to go ahead with it.
To be honest, I don't know her very well. I've only spoken to her a handful of times. She seems nice. Kind of dorky and sweet. Our sexual orientations are compatible, so technically she and I could date, but we have zero interest in each other. What we have (and what your characters have) is called a V relationship. If all three people are dating each other, it's called a triad.
So there are some things you need to figure out.
(1) What do your characters want from each other? Is this just sex, or are they in love?
(2) Is this a long-term arrangement?
(3) The two characters who are friends. Do they want other partners, or are they fine sharing one person?
(4) Did they start off wanting to be poly, or is this a special circumstance?
There's a lot of possibilities, here.