Cal. Sts. & High. Code § 1807

Where the boundary line between two adjacent cities is a street, and the boundary line itself is in the middle of such street, or the side of such street, the two adjacent cities, if authorized by their governing bodies, may enter into a mutual agreement for the improvement and maintenance of the street which constitutes the boundary line, and each city may, pursuant to such agreement, expend funds available to it for the improvement and maintenance of its city streets.