How to open numbers spreadsheet in 2 separate windows

Thinking about this some more, I have figured out a workaround. Though it's not altogether satisfactory, it could be useful in some cases. The workaround is iCloud.

Save the Numbers spreadsheet to iCloud drive, and then open it from a web browser. Open a new browser window and again open the spreadsheet from iCloud drive and you now have the spreadsheet open in 2 windows.

You could also have a local copy open in Numbers, with additional windows provided by iCloud browser windows, though there would be some lag between local changes appearing in iCloud and visa versa.

There is a (clumsy) workaround. From the File menu select Duplicate and a new window will open with a copy of the spreadsheet. Just be care to use it only to read and make updates to the original spreadsheet. When you are done, you can close and choose delete on the copy.