49 C.F.R. § 24.6
Whenever two or more Federal Agencies provide financial assistance to an Agency or Agencies, other than a Federal Agency, to carry out functionally or geographically related activities, which will result in the acquisition of property or the displacement of a person, the Federal Agencies may by agreement designate one such Agency as the cognizant Federal Agency. In the unlikely event that agreement among the Agencies cannot be reached as to which Agency shall be the cognizant Federal Agency, then the Lead Agency shall designate one of such Agencies to assume the cognizant role. At a minimum, the agreement shall set forth the federally-assisted activities which are subject to its terms and cite any policies and procedures, in addition to this part, that are applicable to the activities under the agreement. Under the agreement, the cognizant Federal Agency shall assure that the project is in compliance with the provisions of the Uniform Act and this part. All federally-assisted activities under the agreement shall be deemed a project for the purposes of this part.
49 C.F.R. §24.6