iX100/S1100i/S1100 only support single-sided scanning.
To scan a double-sided business card with iX100/S1100i/S1100, scan the front side, then the back side of the business card one after the other so that both the front and back side data are saved as a single image.
For details:
ScanSnap Help
(Select the model name of the ScanSnap at the top of the page)