WIA Automation Layer 2.0 doesn’t support duplex scan. This is by design.
WIA Automation Layer 2.0 is based on WIA 1.0. The issue is not related to WIA 1.0, but the way automation layer has been designed to use WIA 1.0 for functionalities.
Use native WIA 2.0 or WIA 1.0 (preferably 2.0, if the target OS is Windows Vista or newer) directly to avoid this problem.