That's my point. The galaxy is way to big for 2 guys to enforce the '2 rule'. Especially since the nature of the beast is the apprentice breaking it to take his own apprentice to overthrow the master.
Another failure of the rule would be - no way in hell 2 guys can keep from being wiped out in mass by the jedi. Especailly with Bane going way back. And, isn't Bane old EU? meaning, no longer canon?
And again, if I start learning the dark side, unless i start preaching it to the galaxy - are they just going to sense me and wipe me out? Doubtful. They've got more pressing matters unless I go after them (and I for one am not that stupid
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The rule of two is simple implausible on it's face. Two easy to get wiped out.