The Reason That The Rule Of Two Doesn't Make A Lot Of Sense
For thousands of years, the Sith and Jedi have been at war, fighting to control the balance of the Force. The Jedi seek to protect and defend innocent lives within the Republic while the Sith desire only dominance over the galaxy. By its very nature, the dark side requires great aggression and anger to draw upon, requiring the user to bend the Force to their will. This is in contrast to the Jedi who seek to submit to the will of the Force, living in symbiosis with its will. Over the millennia, many victories have been claimed on both sides, by light and dark, but few events or individuals have changed the scope of this conflict like Darth Bane. Following the fall of Lord Kaan's Brotherhood of Darkness at the Seventh Battle of Ruusan, the sole survivor of the Sith, Darth Bane, forged a new path for the disciples of the dark side called the Rule of Two. The history of the Sith is obscured in shadow and deception, and as such, what we know of them is often regarded as legend, still awaiting confirmation in the current canon.
Growing up the son of an alcoholic, abusive father, Bane had no shortage of childhood trauma. Born on the planet Apatros, in the slums of a mining camp, Bane was quickly pressed into service as a Cortosis miner. The hard labor and unsavory company quickly made Bane a cold, calloused man hungry for power. According to legend, he was offered his chance during the New Sith Wars when the Brotherhood of Darkness, led by Lord Kaan sought to conquer the Republic. Bane was identified as Force sensitive by the Sith and sent to the academy on Korriban before being deployed as a Sith Lord in Kaan's army. However, Kaan, mentally weak from the long, drawn out war, was deceived and destroyed, along with the entirety of the Brotherhood of Darkness, by Darth Bane. The Jedi believed the Sith to be extinct, and Lord Bane was free to forge a new Order.
The foundation of this Order was the Rule of Two. This rule dictated that the Sith Lords would never number more than two. A master to embody the power, and an apprentice to crave it. This would eliminate the rampant betrayal and backstabbing within the ranks of the Brotherhood of Darkness. At the academy on Korriban, several weaker apprentices would often band together to murder a greater master before their training was complete. These misguided apprentices would then destroy one another in an ill-fated attempt at dominance. After generations of this infighting, the Sith and the power of the dark side had been greatly depleted, with ancient secrets dead and buried with the murdered masters. Bane alone saw how these practices would leave the Sith too weak to fulfill their destiny: the annihilation of the Jedi. Bane theorized that if the Sith were to rule the galaxy, their greatest weapons would be the tools of the dark side: deception, mistrust, corruption, and war. By operating in the shadows, the Sith would destabilize the Republic and corrupt the teachings of the Jedi. Over the next millennia, the Sith would do just that, culminating in the execution of Order 66 and the rise of Darth Sidious as Galactic Emperor.
Over the course of this thousand years, the Sith bent and often flat out broke the Rule of Two. The dark side, by its very definition, is rampant with the betrayal and undermining that Darth Bane saw as a weakness in the Brotherhood. Oftentimes the master would train multiple apprentices, secret from one another, in the hopes of pitting them against each other to determine the strongest. The apprentices, in turn, would find their own disciples who were powerful in the Force, training them before the master's death. In Legends, Darth Vader and Darth Sidious simultaneously trained others outside of the Rule of Two. Vader found the son of a Jedi survivor during the Imperial pacification of Kashyyyk and, after killing the boy's father, took him as an apprentice. Brainwashing and corrupting him from an early age. Sidious similarly took a young Force-sensitive girl from her parents and brought her to Coruscant, where she was trained as the Emperor's Hand, his personal assassin trained in the ways of the Force and to skillfully wield a lightsaber.
The Rule of Two came precariously close to destroying the Sith on numerous occasions. Darth Bane himself feared what would happen if a master were to choose an unworthy apprentice, wasting years of training on one who was unworthy of the title "Darth." This unfortunate fate almost came to pass when Darth Maul was defeated on Naboo. With the downfall of the Jedi already in motion, Sidious required an Apprentice to lead the Separatist movement and force the Jedi into open war. Fortunately, a disillusioned Jedi and count of Serenno, Dooku, was eager to strike out at what he viewed as the corruption of the Jedi by the bureaucracy of the Republic. Count Dooku himself violated the Rule of Two on several occasions as he trained two Sith assassins to serve as his right hand: Asaji Ventress and Savage Opress. While these two were never full Dark Lords of the Sith, Ventress did become powerful enough for Sidious to see her as a threat and order her execution.
While Bane's vision of what the Rule of Two would accomplish came close to fruition, the Sith were ultimately unable to eradicate the Jedi. Whether this was due to the limitation of the Sith Lords to only a pair or the rampant disregard for the Rule in later generations is hard to say. Even with the flaws of the Rule of Two and the Dark Side's inclination toward betrayal and murder, Bane's line of Sith Lords were able to all but wipe out the Jedi Order and claim control over the Galaxy for a short time.