Why didn't Elendil call himself King of just one Kingdom, why separate Gondor and Arnor?

[Faethyn]

Why didn't Elendil call himself King of just one Kingdom, why separate Gondor and Arnor and make his two sons also Kings?

[ertr]

Why didn't Elendil call himself King of just one Kingdom, why separate Gondor and Arnor and make his two sons also Kings?

We are never told the exact reasons for why he did that, but a few possibilities come to mind:

The sheer size of Gondor+Arnor made it too big to be effectively ruled from a single place, so he split it into two parts for more efficient governing.
Elendil wanted his sons to get some practice as rulers before becoming kings for real.

[Erionor]

Why didn't Elendil call himself King of just one Kingdom, why separate Gondor and Arnor and make his two sons also Kings?

So ....

Mainly this was due to geographical considerations. When the island of Numenor was destroyed by Eru Illuvatar because the men were a bit naughty (Sauron, being the sneaky bugger that he is, convinced them to try and conquer the Undying Lands, and that was not allowed, no it wasn't precious), the survivors were washed up in different locations in Middle Earth. Elendil landed in the north west of Middle Earth and founded the Kindom of Arnor in Eriador. Isildur and Anarion landed in the south, and formed the kingdom of Gondor, with the twin cities Minas Anor (later Minas Tirith) and Minas Ithil (Later Minas Morgul).

As the two regions were so far apart, with Rohan and the kindoms of the dwarves and elves in between, they became two separate "Kingdoms in Exile," rather than one big realm, and therefore the two brothers became Kings of Gondor whilst Elendil was the King of Arnor. This would make far more sense than Elendil ruling Gondor, as the distance was so great between the two realms.

Anarion and Isildur shared Gondor, with Osgiliath as the capital city. However they had a city each of their own - Isildur had Minas Ithil and Anarion had Minas Anor. When the forces of Sauron took Minas Ithil from Isildur, he sailed to the north to seek aid from his father while Anarion remained to defend the rest of Gondor. Later, after Elendil's death during the War of the Last Alliance, Isildur and his line took up the rule of Arnor in the north, while Anarion's line ruled Gondor in the south.

I think this explains it, but people can correct me if I have got anything wrong

[Aeroniel]

A somewhat similar structure happened in the Roman Empire, when the emperor Diocletian decided the empire was too big for one person to rule effectively. So he created a tetrarchy, with an emperor in the east and another in the west, and a designated successor for each emperor, also with territories to rule. Was a pretty good system, with the expectation that the junior emperors would be selected based on their merit and be trained in statecraft by the emperors. The emperors would simultaneously abdicate at some point. The experienced junior emperors would ascend to the thrones, and choose their own successors to train.

Sadly, the tetrarchy didn't last long. Biological sons of emperors felt they should inherit the thrones, not the designated and experienced junior successors. And so Rome fell into infighting again.

So what occurred with the Numenorean kingdoms in Middle Earth is not so strange when compared to other kingdoms in the days before fast travel and communication.

[Hephburz-2]

Why didn't Elendil call himself King of just one Kingdom, why separate Gondor and Arnor and make his two sons also Kings?

Because it wasn't one Kingdom?

He was King of Gondor, and he was King of Arnor, they are two separate "countries", can easily just compare him to Elizabeth II, who is Queen of multiple countries.

[straytext]

Because it wasn't one Kingdom?

He was King of Gondor, and he was King of Arnor, they are two separate "countries", can easily just compare him to Elizabeth II, who is Queen of multiple countries.

This. Elizabeth is "By the Grace of God, of the United Kingdom of Great Britain and Northern Ireland and of Her other Realms and Territories Queen, Head of the Commonwealth, Defender of the Faith"

The "other Realms" part is just shorthand because it would take a rather long time to list all the places she's sovereign of every time she's announced.

In the past various British sovereigns have been styled (that's the term) as King of England, Duke of Normandy, Baron of Upthistle (I made up that last one).

[Erionor]

Baron of Upthistle (I made up that last one).

I remember the Baron of Upthistle. Spiky fellow ...

[straytext]

I remember the Baron of Upthistle. Spiky fellow ...

Always took his time sitting down. Lots of wincing.

[Faethyn]

The thing is Elendil and his sons FOUNDED Arnor and Gondor.

Why not just stick together, or come back togerther? Especially since I doubt there were hundreds of thousands of people on those 9 surviving ships from Numenor.

There weren't any kingdoms between Arnor and Gondor at the time... Rohan was originally part of Gondor, given to who would become the Rohirrim sometime after the plague. TEregion was gone. There were only Dunlendings between Arnor and Gondor, and they were sparse and living in the woods, not occupying all the lands in-between.

Also, wouldn't it have made more sense for say Anarion to rule one kingdom, Isildur to rule the other, and Elendil to be High King of both? Why was Gondor just the "testing ground" of the princes to prepare one for the rule of Arnor?

[Erionor]

Why not just stick together, or come back togerther? Especially since I doubt there were hundreds of thousands of people on those 9 surviving ships from Numenor.
...
Also, wouldn't it have made more sense for say Anarion to rule one kingdom, Isildur to rule the other, and Elendil to be High King of both?

Because of this:

So ....

Mainly this was due to geographical considerations. When the island of Numenor was destroyed ... the survivors were washed up in different locations in Middle Earth.

It was purely to do with where the survivors ended up after the downfall of Numenor. Elendil in the North, his sons in the South. They didn't have any control over where they ended up, it is simply where the sea carried them. Although most Numenoreans probably died, there must have been a lot of men who landed with the kings, because there were enough to found two whole kingdoms. However, I would expect that it was unlikely they had the numbers to conquer all the lands of Middle-Earth in between Anor and Gondor.

Why was Gondor just the "testing ground" of the princes to prepare one for the rule of Arnor?

I don't think it was. It's just the way fate worked out. Isildur's city of Minas Ithil was lost, and Anarion's was not, so I guess when Elendil died it made sense for Isildur to take over the rule of Anor because he was "homeless." Also, perhaps consciously, perhaps sub-consciously, Isildur may have wanted to get as far away from Mordor as possible with the ring ...

[ertr]

It was purely to do with where the survivors ended up after the downfall of Numenor. Elendil in the North, his sons in the South. They didn't have any control over where they ended up, it is simply where the sea carried them. Although most Numenoreans probably died, there must have been a lot of men who landed with the kings, because there were enough to found two whole kingdoms. However, I would expect that it was unlikely they had the numbers to conquer all the lands of Middle-Earth in between Anor and Gondor.

There was also a whole lot of Numenoreans who had already settled in Middle-Earth. More so in the south than in the north. Dol Amroth was one of several such settlements.
All the lands in between Anor and Gondor is basically just Enedwaith. Gondor extended up to the river Isen, while Anor ended at the Greyflood. (Isengard used to be part of Gondor, and the Tower of Orthanc were built by them, using the same techniques as when building the main wall of Minas Tirith.)