alt+tab not working in Excel (works otherwise)

I use the alt+tab shorcut to switch between open windows. It works for all other applications (even going from another one to Excel), but if I want to switch between two spreadsheets, it doesn't work. Is there a setting that I need to adjust for this? Any guidance will be helpful.

I know that ctrl+tab works, but I'd like to use alt+tab universally. Also, my Tools -> Options -> Windows in Toolbar is checked.

Thanks!!!

Update...If I open two separate Excel windows it seems to work fine. Apparently, it's just an issue when I open two spreadsheets in the same Excel window....if that makes any sense. Still, this could get annoying so any help is appreciated.

'Universally' ALT+TAB takes you between applications, which is why it works when you have instances of excel running.

I doubt you can get it to function differently when Excel is the current application, otherwise you would never be able to move on to the next app.