One of my previous relationships was like this, offline. My Master (at the time) introduced me to another sub. We both liked her. We had discussed adding another sub, and she seemed like a good fit. We spent a good bit of time getting to know her (not enough, as it turned out). When she finally moved in with us, it was a good deal like having a sister. She was, however, and lot more dominant than I am, and he allowed her to explore that side of her personality with me. Our relationship changed from being nearly equals to her practically being my Mistress. There were definitely aspects of the original dynamic that I missed, and would love to find again.
As for jealousy...at the time, I can remember having jealous "moments"...(as in "Man, I wish that was *me*!") but it never seriously impacted my relationship with either of them. However, I can say, with all honesty, that a plural relationship is one of the hardest things to maintain. It requires a lot of trust, and if one person breaks it, the whole thing crashes down.
Good luck, Psy...have fun in the search, and take your time. *hugs*