This is my own headcannon.

Things that would be talked about are:
- What are they?
- Specialisation
- The Split
- After the Split

Generally, reportings of Hybrids in the entity community are very low. If an entity shows itself as a Hybrid, then the entities it usually associated with its primary alter would no longer see them as fully that type of entity. They would usually keep it a secret/ only people close to them would know until it becomes obvious when they Split.

So, what are they?

They are entities that have two 'alters' (sort of like Jekyll and Hyde) and each alter is a type of entity. They would have a 'dominant' alter (the primary alter) and a 'non-dominant' alter (secondary alter).
However, unlike Jekyll and Hyde which is good and evil, the alters are not necessarily good and bad, they are different because of their personalities. They should be seen as two whole different personalities, almost people, and not an entity with two halves.
The most common type is Country-Country Hybrids; however, Country-Entity hybrids are also common. There are rarely any Entity-Entity Hybrids and there has never been a hybrid where the primary alter is a Full-Time entity.

To best describe Hybrids, here is an analogy

Imagine there are two people in a car, one sitting in the drivers seat and the on there in the passenger seat. The person sitting in the driver's seat is the primary alter and the person in the passenger's seat is the secondary alter. The car is the body.

The person in the driver's seat has the control over the car; however, if the primary alter loses control (this may be due to experience extreme forms of emotion or just sleeping), the secondary alter takes hold of the wheel and controls the car. However, driving from the passenger's seat is difficult so as soon as the primary alter can take control, the control would switch to them.

Can the primary alter give control to the secondary alter? Yes, but not indefinitely.

This can be seen as the people in the car switching places so that the secondary alter can control the body more comfortably; however, the primary alter can force itself to take back control.

Specialisation

So initially, the difference between the alters is minimal when the hybrids are young. However, they same way that siblings in the same house hold can be different, as hybrids grow up, each alter can develop different skills and niches.
These skills becomes attributed them, so when it comes to the Split, they would retain any skills they have learnt but would not be able to do the other alter's skill to the same ability.

For example, as the Roman Empire, both his alters could use a spear and a sword efficiently, however the swordsmanship is attributed to Western Roman Empire and the spearman ship to Eastern Roman Empire. So when he Split, Western Roman Empire lost the ability to expertly use a spear, and the same with Eastern Roman Empire with a sword. He did not forget how to wield them, just not to the same ability.

The Split

The Split is when a Hybrid's alters become two separate people. Physically.

(Analogy time...)

Imagine the people in the car could no longer bare each other, so much so that the secondary alter leaves the car and buys a new one. Each alter has a car, hence are in different bodies.

It is the most common route that a Hybrid splits because of not being able to stand each other; however, external events can come into play when it comes to the Split. Additionally, not all hybrids have Split but most do.

Additionally, a Hybrid can force another Hybrid to split. Due to not many reports of Hybrids, it has not been said how this was done but it has been done before.

Most notable Hybrids are:
- The Kingdom of Two Sicilles (didn't split)
- Roman Empire (Split into Western and Eastern Roman Empire)
- USA (Split into USA and CSA)

After the Split

What happens after the Split becomes a competition of 'the survival of the fittest'
The problem is both alters, now just normal entities, cannot ultimately both be alive. Either one or both has to die.
Although this is on a case-to-case basis, all would relive their most prominent memories, either by dreams or hallucinations.
Next they would subconsciously want the other half to die, so their actions would somewhat cause the demise of the other alter. Their personality changes the longer both are alive.
The moment one of them dies, the other goes back to normal.

Author's Note
If you have any questions, ask, as I understand that it could be confusing.

I don't know why I did this to myselfp :')