Options are available under Step #1 Choose Template to either choose a one-sided/two-sided business card
Select one of the Print both sides template options
The following buttons to toggle between the front / back side will appear on Step # 3 Customize screen
If you don't see the option to switch from front / back of the card, you can change to the two-sided template by clicking the blue Change Template button near the top right of step 3 - Customize. This will keep any designs you've already created. If you haven't started designing yet, you can click on the tab for step 2 - Choose Design and then you will be able to select a blank or pre-designed two-sided template.
Here is a demo video on how to design two-sided Avery card products:
When you are ready to print, go to the Preview and Print page