Mining in Windows requires that a monitor or dummy plug is plugged into each video card before it will be seen by a mining program. After you have started mining, you can unplug it, but it will need to be there every time you start mining. If you only have one monitor, you can plug it into one card, then the other. To select which card to modify, click the "Settings" button and select the desired GPU from the drop-down list.
TY! I will give that a go.