At work I use two monitors arranged like this:
I often want to compare two excel sheets by placing them side by side on both monitors. Just maximizing a single excel workspace with two windows in them does not work well, since I would need to drag the workspace as big as the black square.
Opening a new instance via the start menu works, but most of the times I just want to double click on an .xls(x) file and have it open in a new workspace.
One trick to do that is to write a batch file with these contents:
"C:\Program Files (x86)\Microsoft Office\Office14\excel.exe" %1
An improved trick (see comments) is:
start "" "C:\Program Files (x86)\Microsoft Office\Office12\excel.exe" %1.
(This will not spawn the console window).
Place is somewhere where it will not be deleted:
Then go to an excel file, right click on it and select [Open with]. Browse to the file you just saved and select that. From now on all your double clicked excel files will open in new workspaces which can be put side by side.