Private Shared Sub SetDuplexEnabledCapValue()
    Using deviceManager1 As New Vintasoft.Twain.DeviceManager()
        ' open the device manager
        deviceManager1.Open()

        ' get reference to the default device
        Dim device As Vintasoft.Twain.Device = deviceManager1.DefaultDevice

        ' open the device
        device.Open()

        ' get reference to object that manipulates DuplexEnabled capability
        Dim duplexEnabledCap As Vintasoft.Twain.DeviceCapability = device.Capabilities.Find(Vintasoft.Twain.DeviceCapabilityId.DuplexEnabled)
        ' if DuplexEnabled capability supported
        If duplexEnabledCap IsNot Nothing Then
            ' set value of DuplexEnabled capability
            duplexEnabledCap.SetValue(True)
        End If

        ' close the device
        device.Close()

        ' close the device manager
        deviceManager1.Close()
    End Using
End Sub

private static void SetDuplexEnabledCapValue()
{
    using (Vintasoft.Twain.DeviceManager deviceManager1 = new Vintasoft.Twain.DeviceManager())
    {
        // open the device manager
        deviceManager1.Open();

        // get reference to the default device
        Vintasoft.Twain.Device device = deviceManager1.DefaultDevice;

        // open the device
        device.Open();

        // get reference to object that manipulates DuplexEnabled capability
        Vintasoft.Twain.DeviceCapability duplexEnabledCap = device.Capabilities.Find(Vintasoft.Twain.DeviceCapabilityId.DuplexEnabled);
        // if DuplexEnabled capability supported
        if (duplexEnabledCap != null)
        {
            // set value of DuplexEnabled capability
            duplexEnabledCap.SetValue(true);
        }

        // close the device
        device.Close();

        // close the device manager
        deviceManager1.Close();
    }
}