"Always two, no more no less..."
Are there Sith cops that enforce this rule? Some all powerfull Sith Deity that strikes down the one who breaks it? 😉 Remember DV asked Luke to join him at the end of ESB, had Luke taken him up on it what would he have been? An apprentice Sith Lord. Vader's apprentice to be exact. Behind Siddy's back at that.
"Dont you think Tyrannus, if he was the sithmaster, would eventually found out the conspirasy you are suggesting Siddious had."
Siddy didn't find out about DV's conspiracy (making Luke DV's apprentice as per his proposal to sonneyboy at the end of ESB), and wouldn't have until it was too late had Luke taken daddy up on his offer.
"And what do you think would happen to Siddious then?"
He'd have been pasted. But there are no gaurantees that Tryannus would have known, ever. Siddy is a sneaky b*stard, moreso that Tyrannus. And there are a lot of places in a whole galaxy for a Sith lord to hide an apprentice. Keep in mind that everyone seems to have no trouble accepting that Tyrannus is away from Siddy throughout the whole Ep II movie (unlike Maul in TPM). Why, then, would Siddy have had to constantly be at his master's side under his eye? After all Vader was only with Siddy for one movie in the OT. When the apprentice is away there is nothing to keep him from play. Seems to me a Sith Apprentice could get up to all kinds of mischief let to roam too long (I.E. Vader in ESB again)
It's at least possible. Not saying it's how it is, but there are other scenarios than the ones we have taken for granted.