Spyker,
Look at this post in the Plant Design Engineering Wiki:
communities.bentley.com/.../4488.aspx
The key is to set the ECExpression in the order in which you need the tag format to be.
In your case the Calculated ECProperty Calculation would be "this.NAME & this.DESCRIPTION". You can include any number of properties. One note - Properties that are of the Associated Type (Plant Area, Unit, System, Service) require a slightly different method becuase these are "associated" properties.
For calling Plant Area it would be "this.GetRelatedInstance("PLANT_AREA_HAS_NAMED_ITEM:1:PLANT_AREA, NAME:?").NAME:.
For reference look in the OpenPlant_Supplemental_Tagging schema and the NAME properties on the different classes.
I hope this helps,
Tony DeRosa
Bentley Plant