A combination formed by two or more new medium combustion plants shall be considered to be a single medium combustion plant for the purposes of this Directive and their rated thermal input shall be added together for the purpose of calculating the total rated thermal input of the plant, where:
the waste gases of such medium combustion plants are discharged through a common stack, or
taking into account technical and economic factors, the waste gases of such medium combustion plants could, in the judgement of the competent authority, be discharged through a common stack.