I tried the other suggestions but could not find any .xlsb files on my C: drive and did not have a XLSTART folder in any of the user folders. The only XLSTART folder on my system is in
C:\Program Files (x86)\Microsoft Office\root\Office16 and it's empty. My folder settings are set to show hidden files.
I found that turning off the preview pane in Explorer will prevent the second excel workbook from opening.
Does anyone have an idea how to stop the Explorer preview pane from doing this?