Thank you so much for your response!
I am very much a literal, logical person. If someone is to attach a label or term to something I take it as exactly that. I don't think I could attach to term primary to any partner I had in my life. Each person I encounter brings something special and unique to my life, I would like to think I do the same for them. No more or less important, just different. Using the terms primary and secondary indicates a ranking system for me, something I'm very uncomfortable with. Easy enough for me to say though as I don't feel the urge to be married or co-habitate with anyone any time soon.
We also discussed veto power, which they don't believe in. I can't say I believe in it either, I think my approach would be different though.
The spirit of their relationship agreement is in cooperation. So while a situation with a metamour may be difficult, they would work within the spirit of their agreement and talk and challenge themselves to work through whatever difficulty comes up. If the person dating the metamour chooses to stay with someone the other has difficulty with, they would choose to do so in the spirit of cooperation as well. What this means is that both parties would work together to try and ease whatever tension arises. If the situation was particularly sticky, they'd set up boundaries within their own relationship to help them cope with the other relationship. Their aim is to not interfere with each others' relationships while communicating their needs effectively within their own.
I just don't know if this would work for me either. My hope would be that cooperation existed throughout.If I had a partner who had someone important to them in their life, then they would be important to me. If something challenging arose then in the spirit of cooperation we'd all try and work it out together. Easy enough to say though having not been presented with a sticky situation this way.