On some of the online encyclopedias, they say that Darth Maul is a Sith Master. Now I was under the impression that there are only 2 sith to exist at a time since their relationship is so parasitic. It's not the same as being a jedi master where more than one can exist for the fact that their relationship is more mutual. How does a sith apprentice or formal sith apprentice become a sith master? Even up to the Clone Wars, It should be Sidious as the master and Dooku as the apprentice who replaced Maul. How did Maul come to be a sith master?
It seems fairly informal to me. Anyone with enough Sith training can decide to call themselves a master, but until they prove they most powerful its just an empty claim. There is no paperwork to fill out to certify them, the title is yours when you are strong enough to take it.
Usually the apprentice would have to kill the master but Maul took a sideways step that doesnt fit in with the rule of two, so the whole 'might makes right' idea behind the Sith puts Maul and Sidious on a collision course. Whoever wins will be the 'true' master.
Parasitic? The nature of their usage of the force maybe but I never got any hint of the relationship between master and apprentice was as such other than the rules they enforce. The one sith exists in the comics where a lot of sith existed at once with multiple being trained by the same people. At some point I'm sure there were multiple sith before the rule of two so I don't think it's a dire 'way of the force' type requirement where one MUST slay the other. 'rule of two' seems more like a self imposed rule to limit rivals than some form of ascension to the rank of master.
In terms of "Parasitic", I'm saying that they use each other and seek to conquer over the other in order to be the Sith Master and conquer the world. For example, Sidious was trained by Darth Plagueis and Sidious betrays him, killing his master to become the master. The very reason why Darth Bane reduced it to the rule of two when it used to be an army of Sith.