Using More Than One Window in Microsoft Excel

I keep seeing this question being asked, so I can only assume people are inadvertently turning of the Windows in Taskbar option. The problem is usually something like this:

I can't view two separate Excel files simultaneously.

So, go to Tools Options, View tab, and make sure Windows in Taskbar is checked. If that doesn't work, then there is something more seriously wrong with Excel, and you should take the steps toward troubleshooting it.