I read on Wookieepedia that Darth Plagueis was still plotting with Darth Sidious during "The Phantom Menace".
If Plagueis is Sidious' Master and Maul is Sidious' Sith Apprentice doesn't that violate the Sith's Rule of 2?
Too many people seem to take the Rule of Two far too literally.
Wookieepedia explains the rule:
The Rule of Two states that there would be only two Sith at one time, a Master and an Apprentice, guaranteeing that when the Apprentice becomes powerful enough in the ways of the Dark Side to take the title of Master, only then would they be worthy of the title. This transfer of power would only take place when the Apprentice takes their Master's life and finds a worthy student to repeat the cycle. Both Master and Apprentice were considered to be Sith Lords.
But even here, there is a problem. If we follow this rule as described we have the following order of events:
If we take the "Rule of Two" to be so specific as Yoda's quote "Always two there are, no more, no less" and never deviate from this exact specification, the order of events as described cannot ever happen as it defies the supposed rule.
Similarly, if we follow an alternate chain of events (as we see here), we still violate the rule.
But let's go back to the actual conversation Yoda's quote is from.
MACE WINDU : There is no doubt. The mysterious warrior was a Sith.
YODA : Always two there are....no more...no less. A master and an apprentice.
MACE WINDU : But which one was destroyed, the master or the apprentice?
It can be interpreted that Yoda isn't saying that there are only two Sith ever, but that there are always two Sith working together: a master and an apprentice. If one Sith shows up, there is "always" another one involved.
The real takeaway regarding the Rule of 2 should be that Sith are too power hungry to allow more than one other to exist. They need to be the most powerful being. Papaltine/Sidious, however, is more than just ambitious and power hungry, he's also incredibly patient and conniving. He's willing to let things play out long term in an effort to gain everything he wants - even allowing three Sith to exist for a time.
In the novel Darth Plagueis, part of the unfolding plot is exactly your question: what is to become of the rule of two? Plagueis recruits Palpatine as a child and all throughout their training he indoctrinates him with the idea that Bane's rule of two will end with him (Plagueis) and they will be the start of a new galactic order of Sith. This is their 'grand plan', that Palpatine would weasel his way to the Chancellor seat and as his first act make Plagueis the co-chancellor (on equal footing). The rule of two can arguably be said to have ended with Plagueis's own master, Darth Tenebrous, because it is revealed that Tenebrous had another apprentice besides Plagueis, Darth Venemis.
Another thing to consider is that Plagueis knew of Maul since he was a child. Palpatine asked permission to train him. Part of the novel speaks about how Maul was never intended to become an actual Sith lord; he would be merely a tool, and attack dog, of both Plagueis and Palpatine until they took over the Senate. Then they would bring Maul into the public arena, if he made it that far alive. They often spoke about Maul dying during his missions.
The thing to take away is the rule of two is not something that is integral to the Sith religion. It is merely one person's (Darth Bane's) interpretation of how best to master the force. So, to paraphrase a quote from everyone's favorite pirate, it's not really a rule so much as it is a guideline.
Yes, it does violate.
For those who don't understand the purpose of the Rule of Two:
"Grant power growth" means that the power will keep on the same level or rise, never decrease.
Of course there are a bunch of minuses and pitfalls in this approach, but it works and works well.
Also I'd emphasise on some maybe not so obvious details:
The Rule was first implemented by Darth Bane (inspired by ideas of an ancient Sith Lord, if I'm not mistaken).
There must always be two. If you read the Darth Bane series of books, Zannah takes on her own apprentice before Bane is dead. The apprentice she takes on is not a Sith anything because she is an apprentice and cannot have an apprentice herself. So that person, Zannah's apprentice, is just a person who practices the Dark Side of the Force. When Zannah becomes the master. That person becomes an apprentice. I know I'm late on this.
Darth Maul was never a Sith Master, just an apprentice. Darth Bane believed it was a good choice for only two Sith because you never knew if you killed the master or apprentice and most of all, Bane saw how there were Jedi everywhere and it would be easy to take out a group at one time. It was easy to find the Jedi because they were spread everywhere. With only two Sith, it was hard to track them down or know were they hid. You could not take out a group of Sith if they kept their numbers low and Bane learned from passed killings that his idea would prevent this from happening again. I always thought Bane used this as a guide line and not a rule set in stone. The Sith are adaptable to situations and Darth Bane would realize that changes would be needed in certain situations.