Gondor and Arnor a handfull

[doug*platypus]

It seems to me that the two Kingdoms were not sufficiently linked together

Also, the two kingdoms were very much in touch with each other.

Subtle difference in wording, but a difference nonetheless. Much the same as the difference between Kingdom and quite clearly not a Kingdom, but a Co-Kingdom (no matter how the word sounds, that's what Ithilien was if you don't like Princedom). In the end the Palantíri, wondrous as they were, weren't much help in the survival of Arnor.

I can't see what advantage there would be to having a High King, and I don't know why the two realms were not separate. I think Aragorn should have been relieved that Denethor and Boromir were not around to dispute his claim. He wasn't descended from Anárion after all, and the High Kingship seems more like a device for the Kings of one Kingdom to claim rulership over the other. Or could only Arnor claim the High Kingship, as descendants of the elder son of Elendil?