We have a notable point where Sev'rance is described as the 'leader' of the Dark Acolytes and we then see her replaced by Ventress, who herself becomes Dooku's secret apprentice and very prominent herself. Are we to assume Sev'rance, due to her prominence, as head of the Dark Acolytes, as Supreme Commander of the Separatist armies, is just as important to Dooku? He would have had her to hand for the last two years before Geonosis, and he tasked her with securing his own escape route during the battle. Seems likely to me, but also mere speculation on my part. Of course, we then have the question of who Dooku's apprentice was after Ventress, though potentially his play of Sora Bulq and Quinlan Vos against each other (as his last Acolytes), could be interpreted to be him selecting an apprentice, as much as Vader planned to do with Lumiya and Flint... thoughts?
Yes, Dooku explains his intention to keep her close to them and further train her in the dark side, after she takes Sarapin. Sidious knew about her, though, so she wasn't exactly "secret." Of course, she's killed early in the war. Ventress then becomes the chief Dark Acolyte of Dooku's, and Grievous becomes the new Supreme Commander.
Asajj is consistently described as a Sith apprentice by official sources, so while I agree she certainly wasn't a true Sith, she was probably a cut above just being your run-of-the-mill Dark Jedi. She's probably best described as quasi-Sith or something.
If she's referred to as a Sith apprentice by official sources, then she's a Sith apprentice. Seems simple enough. There's no need to complicate things that don't need to be complicated.
They keep throwing the term so casually to the point where it should be disregarded, and that's like saying Savage should qualify as well.
On Asajj, remember that in TCW before she went renegade, Sidious believed Dooku was training her to be his Sith apprentice which was why he ordered that he terminate Ventress or die himself. Really, I think it depends on intentions... not everyone would be a Sith apprentice, its possible Dooku built a pool of Dark Jedi in order to see which one would be an apprentice. I think TCW has shown that he had an interest of eliminating Sidious when he got Savage... not sure if he said that or not but I seem to recall a line of dialogue where he did indicate that he wanted an apprentice to overthrow his master. But how much of that is an act and how much is genuine is up in the air I think...
Except Maul was unambiguously a Dark Lord of the Sith, contrary to what one solitary source would have you (bizarrely) believe.
Maul was a Dark Lord of the Sith, meaning he possessed all the knowledge required to qualify as such. He might have been violating the Rule of Two, but if Maul considered Savage to be a fellow Lord of the Sith, then it's likely that the only thing keeping Savage's title from being totally legitimate was the continuing existence of Sidious and Tyranus. So I'd guess Savage was just as much a Sith as any other apprentice under Sidious was, barring any objections springing from a technicality. edit: Sorry about the double post.
I agree with The_Phantom_Calamari, "There can only be two" is not a scientific fact. The existence of two Sith Lords who both have their own apprentices doesn't mean that the other pair is not Sith. There could be a third group somewhere (like Kesh) and they are Sith as well. When Dooku first met Ventress she declared that she was a Sith. Dooku quickly shot that down but if she never met him nothing would have stopped her from creating her own Sith Order and calling herself the Extra-Dark Lady or something like that.