He didn't fail. He took the Galactic Republic and transformed it into the Galactic Empire, and thus ruled as Emperor for over 2 decades.
Have to remember Sith lords trained others as acolytes, agents, and assassins. Doesn't mean they are Sith (those that receive further training in Sith ideology, skill, and secrets).
Even with the rule of two, Darth Plagueis allowed his apprentice Darth Sidious to train the Zabrak Maul as an assassin. But unknown to Plagueis, Sidious dubbed him Darth Maul, and even Maul didn't know his Master Darth Sidious has a master. Anyway, with Maul's demise, and Darth Sidious's eventual slaying of his master Darth Plagueis, he apprentice's Count Dooku as Darth Tyranus, and teaches him the ways of the Sith.
However, somewhere along the line, Darth Sidious realizes that the anger and strong force sensitivity Jedi Anakin has would be more beneficial to his schemes to unravel the Republic. However, even though Darth Tyranus had served his purpose up to that time, Sidious would allow his apprentice one last test, thus the fight between Count Dooku and Anakin was "set up" so the survivor would become Darth Sidious's future Imperial Enforcer, even at the cost of maybe even not winning Anakin over to the dark side.
Anyway, if anything, even if the rule of two was "bent" a little, in the case of Maul, Palpatine/Sidious I believe adhered to it pretty well.