I have both Product Design Suite Premium 2012 and Ultimate 2013 fully installed with no issues. Prior to my 2013 install I did a 30 day trial install of AutoCAD 2013 Electrical with no issues running Inventor and AutoCAD 2012 at the same time. I uninstalled Electrical and installed Ultimate on the same day.
Hope that helps.
Just remember to save your files in AutoCAD 2012 format. I believe 2013 uses the 2013 format.