Koto K
Raised by a cup of coffee
I think it's fair to say there are different interpretations of the Rule of only 2 sith at a given time. Interpretations between writers, but also intepretations of the rule between characters in the stories/games/etc. For instance- while most feel that the rule of two essentially states- Only 2 Sith can EXIST at one time- a master and apprentice. Another interpretation however could be that- There can only be two sith in any Master/Disciple pairing. One Master, One student. Whereas Jedi Knights/Masters can have multiple padawan learners at a single time, Sith masters can only ever have one.
Personally, I'm more for the second intepretation- Simply because if you look at it in the perspective of the movies- Darth Sideous had Darth Maul as his apprentice in Phantom, yet Darth Tyranus was his disciple in Clones, yet Tyranus was a master in his own right by that point. And clearly, Tyranus being older than Maul, one might assume that Dooku recieved his training prior to Maul, and was already classified as a Sith Lord Master by the time of Attack of the Clones. (though he still considers Sideous his MASTER, which is not unusual since even a Jedi Master like Windu considered Yoda his master). I think Dooku's power at that point illustrates that point. He's nearly equal to Yoda in power and lightsaber handling, while Maul was cut down by a Padawan.
Anyway, that's my thought. Feel free to disagree at your leisure.