|Windows often displays on only one display by default. |
RIGHT click on a blank part of the desktop screen, select Personalize, then Display Settings. Place your cursor over the two displays, one at a time. For the one that shows Not Active or similar, RIGHT click on it and activate it or similar.
If that works, if you leave the USB cables for the monitors plugged in when Windows is running, Windows will retain both displays.
If you unplug one of them and then run Windows with it unplugged, you will have to do the same thing again the next time you plug it in.
If that doesn't help, you need to plug the monitor you are not getting the display on into a different USB port, or you need to use a different USB cable.
Troubleshooting USB device problems
See Response 1: