Liminal characters are basically stuck between two natures, and I have zero ideas on how to write them. Do you know any tips, ideas or general guidelines that can help me?
Liminal Beings are beings that exist between two states of being, not necessarily those that are stuck between two types of behavior (personality not nature), as you have seemed to imply.
First, though, I'd like to address what I believe is causing you to struggle. Your characters, as you've described them, and even the conflict, aren't struggling with an identity crisis, they are one dimensional and flat. You've described archetypes, but not characters. They need depth. They need attachment. It would appear that you are trying to write a story by formula, and it doesn't work like that. Formula is great for laying out the basic bones, but the rest can't be done with that. Your premise (Enlightened vs Romantic) also seems bland and one dimensional - but that, most likely, could be due entirely to the brevity of the mention.
Back on topic.
Liminal Beings are something of a juxtaposition. They aren't stuck between two things so much as they are both a part of both worlds, but not belonging fully to either. They are the ones that live and operate within the borders. They guard the edges, they interact with both parts of their natures, but are often excluded by others and untrusted by those they deal with.
More modern stories also play on them. Mixed heritage beings are often used, half-elves, half-orcs, bastard offspring (as in GoT) etc.
So how do you write them? That depends on the role they play. But you need to give them depth. Feeling. Conflict within themselves as the parts of their nature struggle to reconcile, or conflict with others as they have reconciled their nature, but still find opposition externally. They may incorporate the best aspects of both their natures, but also their flaws, never being seen as whole by either side...Do they rail against their duality of nature? Do they embrace it? Do they work for the good of others? Or for themselves? How are they shaped by their upbringing? Do they have an axe to grind against everyone else, or are they more compassionate for those on the fringes of society because they understand the struggle?
As somewhat of a liminal being myself, I would say the defining conflict for one is to be attached to both worlds, and unable to live fully in one or the other. A typical narrative would be that it tries to commit fully to one side, and then is stopped by something --rejection by that side, friends or relatives on the other side, some intrinsic personality trait from the other side, a sense of guilt, etc. Then it reverses and tries to live on the other side, only to have a new and different set of things pull it back the other direction.
This is an especially rich situation if the two sides are in irresolvable conflict in some way or another. Your liminal character can be a way to explore each side in depth --strengths and weaknesses --and to keep your world from becoming too starkly black-and-white in terms of audience sympathies.