A scanning device scans both sides of a document. The document is fed between a first image sensor module and a second image sensor module. The second image sensor module is moved with respect to the first image sensor module so that a separation distance between the second image sensor module and the first image sensor module is based on a thickness of the document. A first side of the document is scanned using a first scan line located in the first image sensor module. A second side of the document is scanned using a second scan line located in the second image sensor module. When the document is fed between the first image sensor module and the second image sensor module, the document does not reach the first scan line and the second scan line simultaneously.