Robert,
If the two images have the same base name and are in the same folder (or are in different folders but both folders are opened in the same Contact Sheet tab) then yes, both can be tagged (or untagged) at the same time. By default, Photo Mechanic will join the two image formats and show them combined into a single item in the Contact Sheet. The ability to join them is controlled in the View menu.
In order for changes to be applied to both the RAW and the JPEG the two images must be joined (stacked) together. If they are separate then a change to one will not affect the other.
HTH,
-Kirk