This happens usually by accident in the course of clicking around a
sprdsht. You can do it on purpose by clicking ~Window ~New and have to
instances of the same spreadsheet visible at the same time. I use this
a lot to watch what happens to tab A when I make a change on tab B, for
instance.

To revert to the original, simply click the X on xls:2 in the upper
right corner, just as if you were closing the file. xls:1 should now
be visible with no :1 extension. Save this file, and the second window
will not appear until you invoke it again.