10 U.S.C. § 749

(a) When the Army, Navy, Air Force, Marine Corps, Space Force, or Coast Guard, as the case may be, has on duty in the same area, field command, or organization two or more commissioned officers of the same grade who are otherwise eligible to command, the President may assign the command without regard to rank in that grade. (b) When officers of the Army, Navy, Air Force, Marine Corps, Space Force, or Coast Guard are on duty in the same area, field, command, or organization and two or more commissioned officers of different services, who are otherwise eligible to command, have the same grade or corresponding grades, the President may assign the command without regard to rank in that grade.