[Zinnia's note: the original letter writer used an anonymizing system that really confused me, so I've swapped to my own pseudonyms.] I have a 2.5 yrs LDR (Zeus) & a shorter relationship (Hera). Zeus & me decided to try being poly. When I got into a relationship with Hera, Zeus made me break it off. Hera & I still saw each other socially with Zeus's knowledge. Hera broke things off twice because they couldn't handle me being in another relationship. After discussion & research they said they were okay with it. Hera & I put the dating label on it again with Zeus's knowledge. Zeus then gave me a choice: Zeus or Hera. Zeus is okay with an open relationship and a polyamorous relationship later. While Hera says they are okay with all aspects of being poly. Advice?
The person who taught me how to be polyamorous had a motto: "If someone is trying to make you choose, choose the one who isn't."
It does not sound like Zeus wants to be in a polyamorous relationship. When you started seeing Hera, they "made you" break it off. Then, when you and Hera decided to start dating again, again Zeus made it clear that they didn't want to be in a relationship with you while you're also dating Hera.
Perhaps Zeus is okay with an "open relationship," but it sounds like you two have different levels of comfort and sets of boundaries about what that means. My advice is to identify exactly what Zeus wants in a relationship. Then, identify whether that's something you can provide. If it's not, end the relationship.
Then you need to decide if you want to date Hera. Because the choice isn't a binary between Zeus or Hera; you can decide to date neither! If Hera has needs you can't meet or can't meet your needs - if there's too much baggage hanging around from this mess, if you saw red flags in their choice to continue disrupting your friendship over the fact that you wouldn't date them - don't date them either. If you do, make sure you go in with a clear understanding of the terms of the relationship and ensure that you're both operating with the same definition of "polyamorous relationship."