I want to be able to compare to large spreadsheets side by side...
Since you can not natively expand to all monitors I would like to have the two documents open in two instances. These files are links on Sharepoint.
In my experience, if you open the second spreadsheet using File >> Open, it opens in the same program window.
If you open a second Excel instance from the Start menu or a desktop icon or such, then use File >> Open in that window, it opens in its own program window.
I agree that it's broken behavior and I know of no way to fix it except, perhaps, begging Microsoft to change it in some future release.
Seems to be the default behavior for me -- I get a seperate process, even. Try launching excel directly without opening a file to see if you get a seperate process/instance and open your file with OfficeButton->Open.
You can see the simple method at this link, or use this solution:
Go To Control Panel->Folder Options->File Types
Uncheck "browse in same window" in advanced window.
Then highlight Open
Make sure in the Action box it says &Open
Make sure in the application used to perform action it says:
Check the box next to use DDE
Remove anything that is in DDE Message box and DDE Application Not Running box.
Make sure the application box says: EXCEL
And in the Topic box it says: System
I had the same problem with opening files in Sharepoint. The (crude) method I use is: