Was the Rule of Two a Secret?
The RULE OF TWO is one of the most important aspects of the Sith, especially after the Sith's defeat during the fall of the Old Republic. Even before Darth Bane mandated that there could only be two Sith at a time, the Sith of old still operated under an idea of two; a master and an apprentice. The ancient Sith Temple on Malachor (seen in STAR WARS REBELS & Marvel's DARTH MAUL mini-series) had passages and lifts that took a master and apprentice to operate.
It's important to note that Darth Bane's Rule of Two philosophy was discovered by the Jedi Order and other individuals. Yoda, Mace Windu, and Pre Vizsla talk about the Rule of Two as if it is common knowledge.
"There's no doubt the mysterious warrior was a Sith." "Always two, there are. No more, no less. A master and an apprentice." "But which was destroyed, the master or the apprentice?" - Mace Windu & Yoda (THE PHANTOM MENACE)
"Created the Rule of Two, you did." - Yoda to Darth Bane's Specter (STAR WARS: THE CLONE WARS: SACRIFICE)
"I thought there can only be two Sith: a master and an apprentice?" - Pre Vizsla (STAR WARS: THE CLONE WARS: EMINENCE)
Darth Bane's StarWars.com Encyclopedia entry says, "The rule of two was revealed to the Jedi Order of centuries past, but in their hubris, the Jedi believed that they had destroyed Bane's plan with his defeat."
So, we know that the Jedi Order knew about the Rule of Two, but they believed it was destroyed along with Darth Bane.