How to set two-sided printing as default in Mountain Lion Preview?
In the print dialog you should save a new preset. Below you can find the steps to do so.


Open the Print dialog. It will look something like this:

Check the Two-Sided box.

Go to the Presets drop down list and click Save As...

Type a name for the preset in the dialog and choose if you want this preset available for all your installed printers or just this one.

Now choose that preset in the dropdown list. The next time you will print something the preset will be automatically active, so it will print two-sided as default.