Canon PIXMA MG4200 Series On-Screen Manual page 751

Full Platen x 2
Scans the left and right halves of an item twice as large as the platen separately.
Scan Direction
Scan from Left
Displays the first scanned image on the left side.
Scan from Right
Displays the first scanned image on the right side.
Scan Image 1
Start Scanning Image 1
Scans the first item.
Scan Image 2
Start Scanning Image 2
Scans the second item.