There are two techniques, depending on if your scanner has a document feeder supporting duplex scanning.

Check to see if "Input | Mode" has "Duplex Feeder" as an option. This will do two-sided scanning.

If not, then try the following:

1) Scan the front sides of a document to a multi-page PDF file

2) Scan the back sides of a document to the same multi-page PDF file

3) Use the "PDF | Interleave" command

Note that you need exactly the same number of front sides and back sides - there has to be an even number of pages in the multi-page PDF file.