If your image device is equipped to only do simplex scanning (scans only one side of the page), DTWAIN allows you to scan one side of a set or pages, and then scan the other side of the set of pages. After scanning both sides, DTWAIN collates the pages and creates a single multi-page image file (TIFF, Postscript, PDF, or DCX formats).
To use manual duplex scanning, the device must be capable of acquiring one set of pages in a single acquisition (usually a scanner equipped with a document feeder). If the device is not equipped with a feeder, the best option is to use the DTWAIN_SetMultipageScanMode and scan the front and then back sides of each individual page, and then saving to a final multipage file when necessary.
The DTWAIN function DTWAIN_SetManualDuplexMode allows you to turn on this mode of duplex scanning. This function should be called before calling the DTWAIN_AcquireFile or DTWAIN_AcquireFileEx, and the file type must be one of the following when calling DTWAIN_AcquireFile or DTWAIN_AcquireFileEx:
Tiff format
DTWAIN_TIFFDEFLATEMULTI
DTWAIN_TIFFG3MULTI
DTWAIN_TIFFG4MULTI
DTWAIN_TIFFJPEGMULTI
DTWAIN_TIFFNONEMULTI
DTWAIN_TIFFPACKBITSMULTI
DTWAIN_TIFFLZWMULTI
PDF/Postscript Format
DTWAIN_PDFMULTI
DTWAIN_POSTSCRIPT1MULTI
DTWAIN_POSTSCRIPT2MULTI
DTWAIN_POSTSCRIPT3MULTI
DCX Format
DTWAIN_DCX
When using the manual duplex mode, the application must know what type of simplex device is being used to acquire a single side of the set of pages in terms of the orientation of the pages. The application must know whether the stack of pages are placed face-up or face-down in the feeder, and whether the feeder mechanism feeds from the top or bottom of the stack of pages.
The definition of face-up or face-down is highly dependent on the device, but in most circumstances, a face-up feeder usually means that the printed side being scanned is facing the user. A face-down feeder usually means that the printed side being scanned is facing the feeder mechanism.
Face Up 
Face Down
Once it is established whether the pages are placed face-down or face-up, the next step is to determine how the pages are fed into the device. Either the device feeds pages starting from the bottom of the stack of pages, or starting from the top of the stack of pages. Usually, the top of the stack of pages is the page that is facing the user, and the bottom of the stack is the page that is underneath all of the stacked pages, and is the page facing the feeder mechanism.
Given this information, there are four configurations that are possible:
Page 1 is Face Down, Bottom Fed pages -- The device requires the user to place the stack of pages face down (with page 1 at the bottom of the stack), and that the bottom page in the face-down stack of pages is fed first.
Page 1 is Face Up, Top Fed pages -- The device requires the user to place the stack of pages face up, and that the top page in the face-up stack is fed first.
Very rare, but available are the following:
Page 1 is Face Down, Top Fed pages -- The device requires the user to place the stack of pages face down, and the top page in the face-down stack is fed first.
Page 1 is Face Up, Bottom Fed pages -- The device requires the user to place the stack of pages face up, and that the bottom page in the face-up stack is fed first.
It is advisable that the application asks what type of device the user has before assuming the device configuration. There is no way for DTWAIN to determine the device characteristics for whether pages are face down or face up or how the pages are fed.
For consistent scanning, the goal is to always scan the odd pages first (page 1, 3, 5, 7, 9, etc), and then on the second pass, the even pages are scanned (2, 4, 6, 8, etc.). The odd pages are termed side one, and the even pages are side two. Note that when scanning a side of pages, it doesn't matter if the pages for a particular side are scanned in ascending or descending order. DTWAIN will correctly order the pages if the configuration has been set up correctly (face-up/down, feed from top/bottom).
Once both sides are scanned successfully, DTWAIN will collate all of the pages correctly, and create a single multi-page image file in either TIFF, PDF, Postscript, or DCX format. If there is an error in scanning either side of pages, DTWAIN notifications are sent to the application, and the application can respond to these notifications by either allowing side one to be rescanned, side two to be rescanned, both sides to be rescanned, or ignore the error and combine the pages as best as possible.
If there is an error, and DTWAIN is told to combine the pages, the minimal number of matching pages are used. For example, if side 1 has 4 "good" pages, and side 2 has acquired 3 "good" pages, the number of pages that this will produce is the minimum of the number of "good" side 1 pages and "good" side 2 pages (in this case 3 pages is the minimum, thus creating a 6 page image file).
DTWAIN Functions for Manual Duplex Scanning
DTWAIN Notifications and Manual Duplex Scanning
User Interaction and Manual Duplex Scanning
Error Reporting and Manual Duplex Scanning