The Rule-of-Two Sith always struck me as a balancing act between making your apprentice powerful enough to be a useful tool and making him powerful enough to easily overthrow you. This is probably a feature, not a bug, as the entire system seems to be designed to force the ruling Sith to constantly have to watch his back and improve himself, or be replaced with a stronger one - pretty much the Sith ideal, even before the system was implemented.
Sidious was very good at being a Master, as he was perfectly capable of having exactly the right apprentice for his plans, and discarded them with impunity when their purpose was served. Maul was a thug assassin, and he served his purpose before getting offed by a Jedi. Tyrranus was a charismatic face-man and military leader, whom Sidious discarded as soon as his private army had served its purpose and empowered his public army. Finally, Vader was an intimidating, implacable enforcer of military rule, which was pretty much Sidious's whole endgame. He also had much, much stronger emotional control over Vader than his other apprentices, and sought to discard him and repeat that process of sovereignty-through-personal-corruption-and-guilt with Luke, who was ultimately a much more powerful Jedi than his crippled father. Regardless of how much power Sidious gave Vader over the Galactic military, he always had stronger control, both over Vader with his emotional and physical leverage, and over the military itself (he was still Commander-in-chief, after all, and his personal Force meddling was supposedly responsible for much of the Empire's military competence.)
Unfortunately for him and the continuation of the Sith, Sidious's emotional control wasn't as strong as he thought it was, and the Sith Rule-of-Two failed to account for a self-sacrificing kill on the part of the apprentice. And so, Sidious gets thrown into a reactor shaft and his apprentice dies moments later, ending the Sith tradition forever until they needed more villains for the EU.