On the surface it might seem impossible. The units for a Revit project are one or the other, but you can beat this by creating a special Tag family. For this tag start with the Generic Tag.rft template and categorize it as a Duct Tag. Add three labels in a row; Size, Height, and Width. Use the Prefix and Suffix columns to add the open and closed parenthesis. Now select the Height parameter in the Edit Label dialog. Use the icon at the bottom with the image of the hand over the pound sign to change the format by un-checking "Use project settings" and then setting the units to Millimeters. Do the same for Width, and you have a mighty fine duct tag that will show imperial and metric units at the same time.