PROBLEMA: Você deve reiniciar o computador após usar a função ChangeDisplaySettingsEx para anexar um monitor secundário

BOOL AddUnattachedDisplayDeviceToDesktop()
{
    DWORD DispNum = 0;
    DISPLAY_DEVICE DisplayDevice;
    DEVMODE defaultMode;
    HDC hdc;
    int nWidth;
    BOOL bFoundSecondary = FALSE;

    hdc = GetDC(0);
    nWidth = GetDeviceCaps(hdc,HORZRES);
    ReleaseDC(0,hdc);

    // Initialize DisplayDevice.
    ZeroMemory(&DisplayDevice, sizeof(DisplayDevice));
    DisplayDevice.cb = sizeof(DisplayDevice);

    // Get display devices.
    while ((EnumDisplayDevices(NULL, DispNum, &DisplayDevice, 0)) &&
           (bFoundSecondary == FALSE))
    { 
        ZeroMemory(&defaultMode, sizeof(DEVMODE));
        defaultMode.dmSize = sizeof(DEVMODE);
        if (!EnumDisplaySettings((LPSTR)DisplayDevice.DeviceName,
             ENUM_REGISTRY_SETTINGS, &defaultMode))
            return FALSE; // Store default failed

        if (!(DisplayDevice.StateFlags & DISPLAY_DEVICE_PRIMARY_DEVICE))
        {
            //Found the first secondary device.
            bFoundSecondary = TRUE;
            defaultMode.dmPosition.x += nWidth;
            defaultMode.dmFields = DM_POSITION; 
            ChangeDisplaySettingsEx((LPSTR)DisplayDevice.DeviceName, 
                    &defaultMode, NULL, CDS_NORESET|CDS_UPDATEREGISTRY, NULL); 

            // A second call to ChangeDisplaySettings updates the monitor.
            ChangeDisplaySettings(NULL, 0); 
        } 

        // Reinitialize DisplayDevice. 
        ZeroMemory(&DisplayDevice, sizeof(DisplayDevice));
        DisplayDevice.cb = sizeof(DisplayDevice);
        DispNum++;
    } // End while the display devices. 
    return TRUE;
}