In discussing matters on the Episode VII thread (NOTE: that's a spoiler thread, don't follow the quote link if you haven't seen the movie and don't want to trip over unwelcome reveals), I made a comment:
Quote Originally Posted by DrNewGod View Post
...That's something that hasn't been done enough in the canon (ignoring EU): exploring personal relationships within the Sith pairs. Count Dooku's attempt to recruit Obi-Wan suggest to me that is a constant, Darwinian chess match between master and apprentice. With each trying to find a new apprentice for themselves, so the apprentice may determine if they're ready to supplant the master, or the master may determine if the apprentice is still worthy of holding his place.
That got me thinking: how often was the Sith Rule of Two more of a guideline than a law? I didn't read much of the EU, which has been de-canonized anyway, and I haven't seen all of the Clone Wars. I do know that Darth Tyrannus took on an apprentice - Assajj? - while Tyrannus was still apprenticed to Darth Sidious.

Further, Darth Sidious seems to have had two apprentices at the same time himself, or nearly so. During Phantom Menace, Sidious' apprentice is clearly Darth Maul, however, The Attack of the Clones is set 10 years after Phantom Menace, and yet there are indications that Darth Tyrannus, or someone like him, had been at work on Darth Sidious' plans at least 10 years earlier, roughly at the same time as Obi-Wan Kenobi's bisection of Darth Maul.

Somebody hired the Kimino cloners for Darth Sidious, perhaps either Maul or Tyrannus impersonating Jedi Master Syfodious. Attack of the Clones specifically state that Tyrannus recruited Jango Fett for the clone project; now that one's a bit looser, because we don't know how much time elapsed between "Syfodious'" arrangements with the cloners, and the first extraction of genetic material from Jango, but it wouldn't seem like enough time for Darth Sidious to recruit Darth Tyrannus from scratch. Also, mind you, Darth Sidious could not have been free to run around doing a whole lot of recruiting or organizing himself at the time, because at the end of Phantom Menace, he'd been elected Supreme Chancellor. There's not going to be a lot of sneaking away feasible for him.

So it would seem that, at a minimum, Darth Sidious would have had to have Count Dooku's recruitment and training well underway while Darth Maul was still in his service. Thus, Darth Sidious may have had two or more apprentices, more or less, simultaneously.

Was Darth Maul a new apprentice, intended to challenge Darth Tyrannus for his place at Darth Sidious' side? Was the late Master Syfodious another such apprentice? Perhaps even Darth Sidious' main apprentice, who failed in a play to supplant Darth Sidious, or supplanted himself by Darth Tyrannus or Darth Maul? Was Darth Sidious' agreement with Darth Vader to recruit Luke Skywalker understood by both Sith Lords to be a Darwinian opportunity for each of them to test their worthiness to hold their respective titles (and their lives)?

Ignoring the sidelined EU material, what does current canon suggest to you?