Saga Why was the rule of two invented?

Sidious did it. And honestly, how else was it supposed to work? More than one apprentice would make them gang up against their master (which is what led to the Sith's near extinction). One master (powerful and in check) and one apprentice (weaker in comparison and naive) is much more controllable.

 

An awesome, seven-foot BLACK KNIGHT OF THE SITH makes his way into the blinding light of the cockpit area. This is LORD DARTH VADER, right hand to the MASTER OF THE SITH.

--ADVENTURES OF THE STARKILLER (episode one) “The Star Wars” by George Lucas SECOND DRAFT January 28, 1975 LUCASFILM LIMITED.



Suddenly, there is a slight lull in the fighting, then a deathly quiet sweeps over the passageway as an awesome, seven-foot tall DARK LORD OF THE SITH makes his way into the blinding light of the main passageway. This is DARTH VADER, right hand of the Emperor.

--“THE STAR WARS” From The Adventures of Luke Starkiller By George Lucas, August 1st, 1975.


Perhaps a hint that the Emperor was originally conceived as the "Master of the Sith"

 

Speaking of which, I love how in one of TPM featurettes (can't recall which) Lucas refers to the Sith as "Sith Knights", as he once called them on some of the early drafts.

 

I thought that was cool too. I'd love to see that term used if they ever make an old republic film or tv series.

 

LEIA: Father, I think they’re a trap, sent by an evil Jawa trading lord, or the dark master of the Bogan, or maybe even the Emperor himself.

"a clan of Sith pirates, who then spread untold misery throughout the systems. They became the personal bodyguards of the Emperor."

These seem to be references to the Emperor and the Master of the Sith being separate people.

 

At some point they merged - probably after Vader became "right hand of the Emperor".

 

The third draft made Vader the "right hand of the Emperor" and changed the Sith from mercenaries to "the sinister agents of the Emperor." Thr third draft also makes it clear that Vader survives the Battle of Yavin. Considering that the Master of the Sith has the same title and name as a Sith Knight who is redeemed in the first draft and the ending of the second draft talks about a sequel, my thinking is that GL intended to introduce the Master of the Sith in a sequel and ultimately have him redeemed, and that the third draft merged Vader with the Master of the Sith.

 

I wouldn't rule out the possibility that the Master of the Sith was merged with the Emperor rather than Vader, in the fourth draft or so - perhaps after a lot of tossing ideas around.

 

darth bane invented it because the sith were killing each other off too quickly.