The constitution does assume that there will be multiple judges on the Supreme Court. However, it doesn't say that rulings require only a majority vote of the justices. Why? Because it never occurred to the framers that they had to say so. It was such an obvious and common convention that they just assumed it. And if anyone today tried to create a rule that effectively prevented a majority of justices from issuing opinions, they'd be (pardon the expression) laughed out of court.Well, yes, there is a question about that. Drum (and Geoghegan) read the Constitution as saying that normal votes should be taken by majority rules. I think it's just as easy to interpret the record in the other direction: when something important is at stake, better make sure you get it right by requiring a supermajority. As I said earlier, and contrary to Geoghaegan's claims, there is very little in the Federalist papers in defense of Constitutionally-mandated supermajorities (and some of what there is defends them against stricter supermajorities, not simple majority voting). And that's not a surprise, because the entire Constitutional system is built to make it very difficult for simple majorities to get anything done. That's bicameralism; that's separated institutions; that's federalism. That's "extend the sphere" in Federalist 10, and "ambition must be made to counteract ambition" in Federalist 51. In other words...yes, it's certainly plausible that the Framers assumed majority voting in each House of Congress because it never occurred to them otherwise -- but it's also very plausible that if anyone had suggested it, they (or at least Madison) would have loved the idea.
The same is true for Congress. As Geoghegan notes, the framers specifically spelled out cases where non-majority votes were required, something that pretty clearly demonstrates that majority voting was the baseline they were working from. If it had ever occurred to them that anyone would seriously suggest otherwise, is there really any question that they wouldn't have made it explicit?
Of course, my main argument remains that such speculation is unnecessary, because the "determine the rule" clause is straightforward and controlling. But really, if it comes down to the preferences of the Framers, it's very hard for me to see how simple majoritarianism is going to win that battle.