You can apply this setting in two parts of the process:

1. In the editor.

    1.1 Click Preview, on the upper-right corner of the screen. 

    1.2 - Select one of the double-sided options

2. In the print screen (after you've entered your billing information).

    2.1 - Click on Change page layout

    2.2 - Select one of the double-sided options and save.

2.3 -  Regenerate the PDF file to apply the changes. You need to click the regenerate icon:

2.4 - Click the new document to download and print.