The formation of an improvement district under this chapter may be proposed and the petition therefor shall be signed by two-thirds or more in number of the holders of title to the land in the proposed improvement district and the signatures must include those of two-thirds or more in number of the holders of title to the land in the proposed improvement district located outside the district and two-thirds or more in number of the holders of title to the land in the proposed improvement district located in the district if both classes of land are to be included.