Can be:
0 = no support (or error) -> Use TwainGetLastResultCode() and TwainGetLastConditionCode() methods
for diagnosing the error.
1 = 1-pass duplex
2 = 2-pass duplex
Remarks
Before using this method check that the TWAIN state is >= 4 (TWAIN_SOURCE_OPEN)
To get the TWAIN state, use the TwainGetState() method.