Well said, Hyper.
As for jealousy lessening when all are together, I think it could go either way. Being face to face with each other all the time could get them used to the situation, or it could make it unbearable because there'd be no escape. Either way, if they both say there's no way, why even worry about it any further?
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.