Hello PiotrCh,
I searched our knowledge base and found this article for selecting the 2-sided scanning. If you are still having issues after trying these steps, I suggest that you call into our support department for assistance @ 1-800-835-6100.
On my WorkCenter 3345 i can't select double sided scan from Windows App, there is no such opption (Xerox® Print and Scan Experience App).
Double sided scan from printer works fine. Is there any solution to force that option from PC?