(assuming this is for dimieros and autocorrect got you AHA but—) it depends on the AU!

Generally, if they meet each other first (before either of them meet their other romantic partner/s), they will be open to casual sex/courtship but be emotionally exclusive, holding each other as #1. On occasion, they decide to be in an official polyamorous relationship with mihlen’s OC Minkyu/Apfel/Mireu (same guy, dif aliases), who initially is understood to be secondary despite his significance to the two—but after some time, they regard him on the same level as each other so everything is very equal there.

More often than not, though, if Dimitri and Xero are in a polyamourous relationship it’s because one of them is dating someone else first and then meets the other—usually, it’s Dimitri and Minkyu who are an item, meet Xero separately, and then are both like “you know…” about her LMFAOOO In that case Dimitri prioritises Minkyu over Xero until Xero wins over his full trust, and then they’re all equal.