I am working on a workbook with two worksheets. While developing
Sheet1, I need to constantly look at Sheet2. Obviously, this is very
tedious to do in one Excel window.

Ideally, I would like to open the same workbook in two separate Excel
windows. Alternatively, I would like open the same workbook twice in
the same Excel window. Alternatively, I would like to open two
worksheets from the same workbook concurrently in the same Excel
window.

How can I do that?

It would be fine with me if only one instance of the workbook is
writable. The other instances can be read-only. That is, Sheet1
needs to be writable, but Sheet2 can be read-only.