OSX (Yosemite) Make 2 sided printing the default for *all* applications
First save your print settings as a preset:


Open a document, then choose File > Print. Click Show Details.
Choose any print settings you want to save as a preset.
Click the Presets pop-up menu, then choose Save Current Settings as Preset.
Enter a name for the preset settings, then choose whether to use the preset for only the currently selected printer or all printers.


Then make that preset as default: hold the Option (alt) key when choosing your preset, and it will become Default.

Source: https://web.archive.org/web/20131005034126/http://support.apple.com/kb/PH10768