This is probably the most irritating feature of Excel: I open two Excel files and they end up being opened in the same window of Excel! You can see a different taskbar icon for each of them, but they are both enclosed in the same Excel window. This old-school MDI behaviour is completely outdated and hinders the normal usage of Excel in Windows, since it no longer behaves like other applications.
Surprisingly, there is no solution to this! There is no setting in Excel that can be flipped to make it use a new window for every file.
So, when I need to open a second Excel file, I do this:
Open Excel from Start menu.
Excel always opens with an empty spreadsheet. Close this by clicking its close button in the top right corner (just below the window's close button).
Now open the second Excel file using File -> Open.
I now have two Excel files open in their own windows.