(a) If public utilities share in a joint rate the apportionment of receipts shall be just and reasonable. The method of apportionment shall be approved by the commission and the commission may, if it considers it to be in the public interest, establish the portion to which each public utility shall be entitled.
(b) If the commission does not have professional staff to investigate, evaluate, and testify regarding any proceeding under (a) of this section it may employ qualified professional consultants for this purpose at the direct expense of the parties to the dispute and divide the cost among the parties in the proportion of their respective operating revenues before commencement of the proceeding. The cost allocation to each party shall be determined before employment of the consultants and after giving the parties reasonable notice and opportunity to be heard.