There's a whole gaggle(*) of Darths in Dark Horse's Jedi vs. Sith miniseries. Darth Bane is the only survivor of the series; that's where he gets the "only two" idea.
Darth Vader was the last Darth (or Palpatine, if you count his DE & EE appearances toward Darth Sideous. Any Sith that arise after them will be self-trained, and may not pick up on the whole Darth idea.
Like when Kyp Durron called himself a Sith. He had no real idea what it meant, and probably thought that Darth was just a name or title.
(*) What do you call a group of Sith? A flock? A school? An Order? An army?
[ July 22, 2001: Message edited by: BeastMaster ]
"The Beasts know much that we do not." -Ancient Jedi proverb