[Kuruharan]

That is a very good question. And there are actually a few reasons, however it was originally intended to be one big happy empire. Elendil was originally king of the whole shebang. Same with Isildur.
However, the reason why there were two sub-kingdoms in the original set up is because after the downfall of Numenor Elendil and his sons made landfall in different places. They basically just established themselves where they landed.

Stopping and thinking about matters, it probably would have made more sense for them to gather in one place and have greater concentration of power in one geographic location rather than being spread out. But they did not do that for whatever reason.
That was one of the reasons for the eventual split. Anarion and Isildur were the co-kings of Gondor, under their father, because their father could not rule Gondor in far away Arnor.

After the Last Alliance Elendil and Anarion were both dead. Isildur was High King of the Dunadain. He stayed in Gondor for two years teaching his nephew Meneldil the art of ruling. Since Arnor was the senior part of the empire, Isildur decided to rule that directly and leave Meneldil as King of Gondor. Meneldil was a rather ambitious fellow and he was not sorry to see Isildur take himself off to the North, and I doubt if he was particularly broken up when Isildur was killed. That was when he struck out on his own and made Gondor an independent kingdom on its own rather than a junior partner in an empire. Meneldil was able to do this because the new King of Arnor was Isildur's only surviving son, Valandil, was a child and apparently a rather weak individual. Meneldil became a true king, the kings of Gondor were descended from Anarion and the kings of Arnor were descended from Isildur, so after that point it was not really looked upon as being the same family. A view that the Gondorians took when they were having something of a succession crisis after the death of King Ondoher and his heirs. They rejected the claim of King Arvedui of Arnor and picked one of their generals who was of royal blood.

So originally it was an empire, but a rather unstable one. Arnor was too far away for the High King to control the King of Gondor if the King of Gondor became independent minded. The whole structure was rickety and prone to collapse. The separation was inevitable. And as a matter of fact probably re-played itself at some point in the Fourth Age.

[ January 04, 2003: Message edited by: Kuruharan ]