26 CFR 601.508: Dispute between recognized representatives of a taxpayer.

Where there is a dispute between two or more recognized representatives concerning who is entitled to represent a taxpayer in a matter pending before the Internal Revenue Service (or to receive a check drawn on the United States Treasury), the Internal Revenue Service will not recognize any party. However, if the contesting recognized representatives designate one or more of their number under the terms of an agreement signed by all, the Internal Revenue Service will recognize such designated recognized representatives upon receipt of a copy of such agreement according to the terms of the power of attorney.