You should definitely start one of your own and let your voice be heard outside of someone else's thread, but you're more than welcome to comment here.
In practice, my partner has what might be considered two "primaries" and one "secondary" partner, but he doesn't categorize things that way. His new GF is married, not "out", and has a very busy life, so she doesn't have the ability to have the same kind of relationship that I and my other metamour do with him.
For sake of simplicity, I'll use Metamour 1 (M1) to be his other 'primary-ish' relationship, and Metamour 2 (M2) to be his latest relationship. I will remain 'I' and 'me'.
Partner will be P. It makes the wording easier.
More later - P is arriving soon with his daughter, and we'll be finding some air-conditioned activities to do with all the kids before breaking out the sprinkler and water balloons.