When did the practice of having multiple series for a world begin? I have seen at least two manga where there are multiple series told from the points of view of different characters, but all set in the same world and at or around the same time.

First is chrome-shelled-regios, which includes:
 - Chrome Shelled Regios
 - Chrome Shelled Regios: Missing Mail
 - a couple of others

The other is code-geass which includes:
 - Code Geass: Lelouch of the Rebellion
 - Code Geass: Suzaku of the Counterattack
 - Code Geass: Nightmare of Nunnally
 - a couple of others

Where did this originate and what was the first manga/set of manga to showcase this?
It started at least 2000 years ago with works like Homer's Illiad and Odyessy, and lost works, Hesiod's Theogony, and later, Virgil's Aeneid, among other things.  

Most notably, the Illiad showcased the seige of Troy, focusing mostly on Achilles's role and his dispute with Agamemnon.

The Aeneid focused on Aineas, who fought in the Trojan War, and escaped Troy to found Alba Longa, where the founders of Rome were from (they presumably descended from Aeneas).

It is also known that there are lost works about the Trojan War, which describe other parts of the war such as the sack of Troy.