Why won't Windows Excel let me have two files open at once?

You can have two Excel files open at the same time, it’s just that they’re both represented by the same Explorer tab (or whatever browser you’re using). To see two files side by side, you have to use the Restore Window button for each of the files, not the Restore Window button for Excel. So, while you’re used to looking for the RW button at the very top right corner, instead look for the smaller one just below that. Then you can resize each window so that they can be seen side by side. The same thing goes for the Close button. Don’t look for the one at the very top right – that will close all Excel files. Instead, look for the smaller one below that, which closes only the file in question.

I agree that it’s annoying, and I don’t know why this method was adopted for Excel but not for Word, for example.

You could, if you wanted, open another instance of Excel and have the two instances running side by side. I’ve done that any number of times for years.

Alternatively, within the same Excel session you can go to the View selection on the menu and “Arrange All” to review any number of open spreadsheets, either side by side, stacked on top of each other, or tiled, as you prefer.

This drove me nuts for a while. I finally found out that if you already have Excel open, you can find another Excel file in Explorer, right click on it, and select Open with Excel. That’ll open a whole new instance and you can see them side by side.