I think yoda's status as one of the most powerful Jedi ever pretty much precludes the possibility of him ever having been approached by the sith. A Sith lord would have a much easier time turning a younger, less experienced Jedi, or one who clearly cannot maintain total control of his/her/its emotions. Personally, the rule fo 2 makes sense for the prequels, but I think the sith would abandon it as soon as they'd taken over. It's whole purpose was to allow the Sith to survive after the Jedi destroyed them. once the Jedi are gone, the only threat to the Sith's existence is gone, and the rule of 2 can be abandoned.