I think the writing would be more on the straight forward side. If anything, the Twins would just be their selves, maybe just slightly more smug and pleased than they already tend to be, perhaps more still, they would be more inclined to be forthcoming and open around the two if they both were mutually enthusiastically lapping at the twin of focus's feet or such as the relationship developed. From what I've seen so far from the writing, I don't think it would be at all difficult writing dialogue for it.
Coding wise, it shouldn't be that troubling, it would likely be a similar condition to what is current, just instead of 5 influence and <50 Rowan relationship, it would be 5 influence and >50. At worst, they could make it into an optional trigger you have to select into if they ran into some sort of multi-option circumstance.
I believe the lion's share of the work load would actually be in the art. Making versions of romance scenes that include all three of them would most likely require new art to fit the two potential trios for their various interactions. For it being worth it or not, it would depend on how interested the team would be in developing another branch for each twin.
Without a doubt though, there would be many people who'd enjoy these routes, as it would be a rather common interest among players to both want to pick options that are positive between Rowan and Alexia, while also wanting to pursue a favorite twin. I myself, prior to looking up a walkthrough, wanted to pursue strong relations with Jezera, but naturally was inclined to choose positive options between Rowan and Alexia, as I had no prompt or inherent motivation to purposely try to worsen their relationship. Without outside intervention of a walkthrough (many people will likely want to play the game blind), I would of locked myself out of what I wanted with Jezera. I don't think I would be alone in ending up in that frustrating situation, especially when the game is officially launched post development completion, where many people will be going into the game without such stipulations being exposed to them in advance.
It being similar to Greyhide content in the sense of it being a polygamous relationship as well shouldn't really matter. Polygamy isn't just a kink, but a legitimate relationship structure choice. One that would fit quite well with either twin. The context of those involved matters significantly as well, in which, the context of the the twins' relationship to Alexia and Rowan has a much different feel and context than that of them and Greyhide. I would imagine it being both euphoric with strong sub/dom overtones, as well as high in energy and openness/transparency.
In short, it would be a strong, well-fitted addition to the branch options with the twins from what I can see. With that said, even if the devs showed interest in this, and when presented about it, got positive feedback from fans, it would need to be balanced with all the other things already planned for the game. But, if they did want to expand on branch options, I'd imagine such would be a rather strong candidate on all fronts, especially since it's a route many people will likely end up falling into regardless with their inclined choices. That is to say, many will end up naturally picking positive choices between Rowan and Alexia while also gravitating towards one of the twins. The shoe just sort of fits in this otherwise vacant avenue.