In the case where both monitors are functioning normally in Windows 7 but only one monitor is working in Windows 10, there are a few steps you can try to resolve the issue.
First, ensure that the second monitor is properly connected and powered on. Then, go to the display settings in Windows 10 and click on "Detect" to refresh the display configuration. If the second monitor is still not working, try updating the graphics driver for your system. Visit the manufacturer's website and download the latest driver for your graphics card.
If the issue persists, try swapping the connections between the two monitors or using different cables to rule out any potential hardware problems.