HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

I'm interested in what other types of splits there are... Saves with different examples would be VERY helpful.

There are a few saves in the asm topic:
/topic/5927/

I found two more saves.
In one (game between Kluka-Piterius), there is a typical example of a hero split in a Castle (Deemer in one of the Confluxes of the red player).
In the other (game between Kluka-DubrovinPoll), the secondary structure – the Conflux – turns gray, accompanied by a hero split.