There is not an option within the TaxSlayer Pro Desktop or ProWeb programs to default printing to both sides of the paper as this is not a standard way to print tax returns. Double-sided printing can only be configured through the printer itself.
With the PDF version of the document open to be printed open in Adobe Reader, select File > Print:
The Print Menu will display on the screen. Click the Print on both sides of paper check box, then select Print:
If you aren't seeing the check box, the printer selected at the top is not capable of printing on both sides.
Note: The IRS and state taxing authorities do not accept mailed paper returns that are printed on both sides of the paper. This option should only be used to give a client a copy of their tax return that WILL NOT be mailed to the IRS or state.
Additional Information:
Desktop - Troubleshooting Printing Problems
Desktop - Editing / Customizing Individual Print Options
ProWeb - Creating Custom Print Sets