My dual monitors worked in Windows 7 without any problem. However, after upgrading to Windows 8, I can only duplicate the display or show it on one monitor at a time. It will not extend the display. When I select "extend the display" the primary monitor continuously blinks on and off, and the secondary monitor goes into sleep mode. All of my drivers are updated.

As per the description it looks like you are unable to connect dual monitors on Windows 8.

This issue may occur even if the display resolution settings is not supported by Windows 8 or due to display drives issue.

I would suggest you to make second monitor as the main monitor and check if you are able to view the display.

I would suggest you to refer the following Microsoft article.

Connect multiple monitors to your PC: http://windows.microsoft.com/en-IN/windows-8/how-connect-multiple-monitors

Use multiple monitors: http://windows.microsoft.com/en-IN/windows-8/multiple-monitors#1TC=t1