At any rate, back to the original topic, I get that the Sith were supposed to use cunning rather than brute force to eventually seize power, but a two-person cult is inherently fragile and due to the way treachery is a core part of Sith philosophy, it would be wildly unstable.
Think about this example from the PT: Palpatine killed Darth Plagueis IN HIS SLEEP! He didn't prove himself stronger or anything, he just took a cowardly, dishonorable way out because he has no principles other than power.
Palpatine was not cowardly and did prove himself stronger: His master showed weakness by letting his guard down in sleep.
Even Jedi keep a part of their mind awake while sleeping to alert them to danger.
Plaguise was showing weakness by not being aware, therefore he was weak and no longer worthy of lordship.
As for dishonor, to quote Darth Bane, the Rule of Two founder, "Honor is for fools." The Rule of Two follows the logic, "If I am able to kill my master (in any way at any time), then my master is weak and unfit for lordship."
Now let me ask you: how do you form a teacher/student relationship when both sides by their nature play by no rules other than a selfish desire for power? There's no trust or loyalty established, and you don't even have to announce your intentions when it's time to challenge!
Trust and loyalty are Jedi traits. The apprentice serves only one purpose in the mind of the master, to carry on his legacy. Likewise, the master serves only one purpose to the apprentice, to be a source of knowledge from which to learn from and gain personal power.
Once the apprentice has learned all they can, the master is discarded as he is no longer useful and an obstacle to personal power.
You don't have to announce your intentions because a true sith master is always on guard, expecting the betrayal. Once they let their guard down, they prove themselves unworthy of the title.
What if you're practicing lightsaber techniques, and your Sith master says "OK, fight's over, good job," so you lower your lightsaber and he uses that opportunity to cut you down where you stand?
They would not do that. A master wants the apprentice to succeed in order to carry on his legacy. Even if they did do that, it would prove that the apprentice was unworthy because they let their guard down and were unaware, as I have said before.
Keeping your guard up 24/7 is a core principal to the Rule of Two. Its what makes the rule functional.
See what I mean? And yet Dooku in ROTS and Palpy in ROTJ are both SHOCKED when they're betrayed! Why? Both should've been like, "oh well, that's the way our stupid order works."
See why I say no group could work like this? The Sith are morons.
Darth Bane was many things but he was never proud nor did he let hubris hinder him. Pride, arrogance, and hubris were major sins in Bane's view and Palpy embraced these attributes.
One could say that the reason Palpy's reign only lasted 40 years after 1000 years of planning is because he abandoned these principles and got full of himself, thereby tainting the Rule of Two.
You should read the Darth Bane trilogy it explains this stuff better then I can.