Is there a “Rule-of-Two” for Sith Lords?

This came about after reading a forum on Here’s my thinking:
1. Darth Tyrannus is a pretty old guy — an ex-Jedi. So he’s been around for a while. He’s been around since Darth Maul was around, and maybe he wasn’t a dark Jedi while Darth Maul lived, but it’s more than likely that he was jaded at the same time Darth Maul was killing good-guys. That would make three Sith(s). Or at the very least, two Sith(s) with one “on deck.”
2. People may be generally misinterpreting Yoda’s statement to mean “only two at any given time,” rather than the more plausible explanation of “for each master, there is one apprentice,” such as is the case with all Jedi (i.e. Obi-Wan and Qui-Gon). This would open the realm for more story potential, and would also explain why every time a Sith is killed, another one seems to spring into action.
3. There may also be a possibility that not all Jedis are good, and not all are Sith (i.e. no Yin-Yang). This jibes with the whole Lucas Arts “approved” Jedi Knight II game, suggesting that you could be a disgruntled ex-Jedi (like Kyle), or a dark Jedi (not necessarily a Sith).