As CDV said, you need to create a new dimension type and apply a complete second set of dimensions as metric or imperial. Best duplicate (create) another view to place the second set of dimensions or you can place them in the same view with the main (imperial or metric) dimensions. Revit does not have the ability to display main and alternate (imperial and metric) dimensions with in the same dimension type (Styel in AutoCAD) as AutoCAD. The next release, Revit 2009, will provide more flexability in having additional text associated with the dimension line (above and below) but still not support dual dimensions within the same dimension string.
Mel Persin, AIA
AEC Technology Consultant
Technology to Visualize and Realize Solutions
Modeling the Future/Drafting the Past