There is a known issue that will cause a secondary display to not be accessible to MediaShout. It involves the communication channel between the computer and display that is known as DDC or DDC2. If the computer can't discover any information about the device at the end of the video cable, MediaShout will likely refuse to use it. You will be able to determine if this is an issue by looking in Windows Display Properties to see how Windows labels the device. If it is called by its actual device name, or as 'Generic Plug and Play' or 'Non-Plug and Play', then it should work. If Windows calls the display something like 'Default VGA monitor' or anything with VGA in the title, that is an indication that DDC is not working and Windows isn't certain what the capabilities of the display are. The reasons that this happens are varied, but it usually involves the VGA signal being routed through devices that do not support DDC such as splitters, cable extenders, scan converters, etc.
To test if this is the issue, please connect the VGA display device directly to the computer and see if it is available in MediaShout for Dual Screen mode use. Also try changing the cable to a known good cable with all 14 or 15 pins present. (Pin 9, toward the middle, is frequently not present -- so don't worry about that being missing. It is pins 11 and 12 that we are most concerned with. See this article for more information about DDC and how it works.)