Dark Shadows Mod has many changes here are some of the most important changes-
New Faction - Arnor replacing Men
Renamed Faction - Wild replacing Goblins
New Units
New Heroes
New Colours
New Maps (south dunland and ea's white mountains)
the mod also comes with the patches if your on the wrong patch. the patch needed is in the readme file
please use the patches from here
http://download122.mediafire.com/fyycuz18jysg/axyjqhwmhzy/bfme2+1.03.zip
its a very simple way
and yes you can play the original game at the same time.