Book of the Sith and Jedi Path confirms they found out from some cultists or something about Darth Bane, but they also believed after Bane re-established the Rule of Two, the Thought Bomb went off and killed his ass as well.
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relentless1Dark Overlord of KMC
to be fair, the old republic stuff isn't canon, so feasibly the Sith could've always been just two people at a time since the beginning of the Sith in canon anyways.