Please, review topics below to find answer to your question

File | Open menu command

This popup menu has 2 sub items: Left Pane ... and Right Pane .... Each of them opens Excel workbook. First command opens workbook and displays it on the left pane. Second command displays opened workbook on the right side.

Q.What if I want to compare worksheets of the same file. Should I open it twice?

No. You can select left file on the right pane using file selection Combo Box. But if you open same book twice - no problem will happen.