Ventress was never a Sith, neither was Savage...they were 'dark jedi' apprentices of Sith Apprentices not Sith themselves.
Darth Bane's post Ruusan Sith reformation Rule of Two: Two there are; a Master to embody Power, an Apprentice to crave it. No more no less.
TPM - Sidious, Maul
AOTC - Sidious, Tyranus
TCW - Sidious, Tyranus
ROTS - Sidious, Tyranus then Sidious, Vader
ANH - Sidious, Vader
ESB - Sidious, Vader
ROTJ - Sidious, Vader
Dark Jedi don't count as Sith - Starkiller NOT Sith, Mara Jade NOT Sith, Ventress NOT Sith, Savage NOT Sith because they were neither the Master, nor the Apprentice of the Master.
Maul however WAS the 'official' Apprentice of Sidious - so is real Sith - except that Sidious thought him dead so took Dooku as his new Apprentice which created a POTENTIAL problem for the Rule Of Two when Maul turned up alive.
Notice that Sidious did NOT attack Maul immediately...it didn't matter to him if Maul and Savage ran around causing trouble for the Jedi Order, Separatists or Republic as 'dark jedi' anymore than it really mattered that Tyrannus trained Ventress or Grievous, or that Vader trained Starkiller...
Its only when Maul DECLARED himself and Savage to be Sith 'publicly' to the Galaxy by saying it to the Mandalorians and Black Sun that Sidious then HAD to act directly. If he hadn't then it would have put Tyranus authority in doubt [if Sidious first Apprentice is still Sith under Rule of TWO then how can Tyrannus claim to be a THIRD Sith?] and worse by declaring himself AND Savage Sith he is directly challenging Sidious' claim to being Master...for that Savage had to die and Maul had to be PUBLICLY 'chastized' and 'stripped' of any legitimate claim to being a real Sith to reaffirm Sidious' position as Sith Master, Tyrannus' as his official Sith Apprentice, reaffirm the primacy of the Rule Of Two and prevent 'devaluation of the Sith 'brand' to the Galactic public.
Which was the point of that Sith versus Sith duel.