I think it will be elaborated in the side stories
Seth has to spend some time in langeres because he’s the leader and I’m pretty sure the other kid goes with him now that he’s old enough to learn the ropes because he will take over once he’s of age. Seth said so himself that it’s not that far so they probably stay a few days at langeres then a few days with Theo
They are together but because of there roles as leaders of their family/people they have to live separately. It's kind of like a long distance relationship because of jobs and they live in a different countries to compare it modernly. At least two of their children has to be chosen to be successor of Langeres and Perserus, which is probably how they're gonna retire and finally get to live together. That's why Theo was worried he would miscarry too, because Seth wouldn't have a successor.
I thought they would just said screw lines of successors amd live together withe kids.
I would have like to see what the twins looked in egg form or as babies. And I didn't really understand the ending they're together but live apart? And what the darker child goes with the silver haired dad and come to visit whenever??