The 12th amendment does supersede the language of Article II, Section 1, Clause 3 but the point is still the same... the 2/3 quorum only applies to the electing of the President if there were an electoral tie. This purpose is to make sure Congress is duly represented before the vote can take place if this were to occur, again, since there is unlikely to be an electoral tie when those votes are cast in December, this tactic does not apply at all. Even if there were a tie though, the 2/3 quorum, again, is to make Congress wait until enough people are present to vote, not a tool that could be used to avoid a vote. The electors could simply be ordered by the courts to complete their Constitutionally mandated duty to cast their vote, as specified in II, 1, 2.