It might also be worth remembering that the Rule of Two is in and of itself derived directly from the Jedi Code (twisting in intent though it may be.)
Right off the bat in TPM Lucas established that the code forbids a Jedi from taking on more than one apprentice at a time. Since the Sith were originally Jedi, it stands to reason that they retained the practice along with the robes, holocrons, temples and lightsabers.
While for the Jedi it seems a matter of principle, for the Sith it seems much more pragmatic; take on more than one apprentice and sooner or latter, they'll start killing each other, or worse, join forces and kill you (then each other.) Not having an apprentice at all would be even more dangerous since a lone Sith Lord would be seen as weak and vulnerable by all the others, who would make it their business to eliminate them, then steal all the powerful secrets they're obviously hording for themselves.
At first that worked to an extent with Sith pairing off, setting up with own Empires, or fiefdoms within a more powerful Sith's Empire though alliances of convenience, playing power games. Circling each other like wary scorpions. Each waiting for an opportunity to get their retaliation in first, before it all inevitably collapsed and Bane narrowed the definition of the rule from "only one apprentice for a Master at a time" to "only one apprentice and a Master at all"; a cult of two.
This is why the idea of Sith Academies in Legends never made much sense. You'd never get that many Sith cooperating in such close proximity to each other. Now, Academies run by specific Sith Lords to train/indoctrinate assassins to serve them and them alone on the other hand, makes perfect sense, and that is exactly what the Fortress Inquisitorius really is. A resource to exploit for sure, but they're never going to be getting any good apprentices from it since it's designed to trap the weak, not the strong. If by chance some truly promising candidate turned up in such a place, any Sith with half a brain would get them out of there fast so they can be trained and indoctrinated separately (as Sidious did with Maul.)
Somewhat related side note: I know a lot of people question how Yoda could know about the 'Rule of Two' & Bane, but I find that stems from the convoluted version of events from the EU novels. From what Lucas has said over the years, and what appeared in the TPM novelizations (based on Lucas's notes), the idea wasn't that Bane and the Sith Order vanished for good when all the others died. Instead he went into hiding *before* the collapse, sat out the slaughter in hiding (no need for an elaborate super-weapon mass genocide plot) *then* re-emerged as the lone survivor once most of the others were done killing each other, and the Jedi had picked off the rest. It wasn't until he was near the end of his life that he took an apprentice and established the new way of doing things.
So it seems far more likely that the Jedi were well aware of Bane and that it was his Empire (of that of his inheritors) that they finally overthrew, possibly centuries later. Indeed I like to think that it was the Rule of Two that actually allowed them to take over the galaxy the first time, because prior to that they were spending most of their energy on in-fighting, and their vendetta against the Jedi. With just two Sith they could accumulate far more power and control.
Incidentally I've also seen
interviews with Lucas early on where he said there was never any war between Sith & Jedi, that that was an invention of the fans (in which I assume he includes the EU authors which really tells you what he thought if it all) and the demise of the Sith was their own doing. Given what happened in TCW, he seems to have later changed his mind in that though. Good thing; since what the hell else would the Sith need revenge against the Jedi for?