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.