@Lililily
Marriage is a political contract of high grade. Ok, you may call it custom, but if whole medieval society it moved by such customs, that are solidified by legislation, those are laws.
That's the case of the lands under single family - royal family. In case of Eque and Nix, there are two families who will either have claims on two separate duchies or one of them if Eque marries in. And if a relative from one land have claims on another... Well, hundred years war happens.
Sure, Nix has no offspring, but would he really abandon his own duchy to marry in Eque's family?
Inconventional way is Eque actually becoming female family head in fact. Fran inheriting the land is exactly what Ryan wanted.
Two married people with separate family names leading two states would make them not a married people.
Prince of Pama is pretty self-contradicting character. What he said to his father suggests he gave up, yet still not willing to stop pointlessly courting Tanya.
Yes, this is the same problem as with dukes and it would demand the change of political system to resolve it.
Uniting two family leaders = uniting two families = uniting two duchies = defeating the purpose of having duchies. Probably threatening Tanya's interests in the process. I mean, maybe there is some historical justification that would allow the reshaping of the kingdom, but for noone to mention it to this point is would be huge copout and will ruin consistency of the series.