Hi
As far as I know you are regular contributor in SD forums. Right? Why didnt you post this query in SD forum? This is not the right place to ask such question and there are no SD experts here. If moderator sees such questions here they remove it. Now lets see if moderator is going to remove it or move it to SD forum.
Coming to your query, I think this is not possible from only configuration side. You need to create a Z routine in VOFM and assign that for discount condition type. Ask your ABAPER to write logic that if there are more than one TAD item categories then this routine will be activated or otherwise XKWERT = 0.
Thank$