That's exactly why it potentially WOULD cause conflict
Imagine swearing fealty to the Queen of Sweden and also the Holy Roman Emperor at the same time
More like King of Sweden and one of the many princes that rules a tiny part of Northern Germany nominally subjected to the HRE - both Boer'alvar Queen and Carmen seem to have very modest holdings compared to even the successor states of Belhar. In any case, such situations happened quite a number of times throughout history, especially if the land owned by the vassal is on the border between the lands of the lieges. Not least because both titles can be held by the same guy.
Speaking of which, I hope that we can eventually get an option to convince Carmen to abdicate and give up the lands she technically owns to either us or Ryn (or formally free them to self-govern). Since I now like her quite a bit more and no longer find her
that sus, she can get a nice pension out of it and all the virgin D's she needs.
Edit: Oh, and Kitsunes are to be yeeted out of our new kingdom, unless they are willing to pay good money for the privilege of establishing a dedicated trading settlement or two. I'd feel bad for Kohaku and a couple of others, but one shouldn't let personal sympathies get in the way of policy making. And unless their drastically change their attitudes, kitsune colonists are not good neighbors, simple as that. Evergreen will need to be rooted out ASAP for the very same reason.