You can open two Excel files in same instance each taking half of the screen (View - Arrange all - Tile), see first screenshot attached.
Or you can use command Run as
in elevated command prompt
or create a bat file with this command.
Runas /user:Guest "C:\Program Files\Microsoft Office\Office14\EXCEL.EXE"
Of course instead Guest you can use any other user created in Control panel - User accounts.
In this case file will be run as a different user (see second screenshot - Guest vs. Admin).