MacOS – How to set two-sided printing as default in Mountain Lion Preview

You might want to check out this question: What window management options exist for OS X?

My recommendation, if you want it to be fully automatic, is to use Slate.

Slate is no longer in active development, but it still works and is highly customizable. Put this code in a .slate.js file in your home directory:

var rightHalf = slate.operation("move", {
   "x" : "screenSizeX/2+screenOriginX",
   "y" : "screenOriginY",
   "width" : "screenSizeX/2",
   "height" : "screenSizeY"
});
slate.on("windowOpened", function(event, win) {
    if (win.app().name() === "Preview") {
        win.doOperation(rightHalf);
    }
});

If you aren't printing very long documents, one option would be to manually deselect the even/odd pages in the Print dialog.