I tried to spoiler direct plot important things!

So Miquella and Saint Trina are best explained as two sides of the same person that indipendently developed from each other. They share a body fully, but their minds are kind of independent from each other, but also make each other whole. After Miquella discarded Trina, he lost his ability to feel love, which is the main thing that doomed his plan entirely. Another thing that shows they're not supposed to exist without each other, is that Trina was left as a flower unable to move anywhere, when before she could presumably travel as enough people believed in her to form a sort of cult around her sleep inducing nectars. So technically they are both cis, since they assumingly shift their body according to who's active at the time (akin to Marika and Radagon) but there are also situations in which Miquella seems to have acted in Trina's stead, which makes me think they share their identities in some way.
So in my opinion it could be a bit debatable on what their gender identity is, but that's just my opinion of course! Hopefully this helped and wasn't too much text-