For me, most of the windows can alt-tab back and forth just fine.
But somehow this is not the case for Excel.
When I tried to alt-tab between Excel and another window, it would switch alt-tab between the 1st & 2nd prior windows that I clicked and not the Excel sheet.
I accidentally found a quick temp fix for this.
I minimized the excel that I was working on and opened it again and found alt-tab is stuck to Excel itself.
I then open the window that I want to alt-tab between by clicking on the task bar.
This allowed me to continue alt-tab between my excel and the program that want to alt-tab with.
Not sure if this would work for anyone else but just thought I would share.