Open the first document.
Open Excel (do not double click on a file, just open excel like normal where
it brings up a blank document)
You should now have two instances of excel open.
Open a file through file->open
Also, if you just want to split documents in the task bar (documents opened
in the same instance of excel are usually stacked in the taskbar) click on
Tools->Options->View->Check the box next to "Windows in Taskbar". You will
notice that the file windows are still inside the one excel window.