Mont. Code § 81-4-304
Herd districts may be created jointly between any two or more counties of the state of Montana where the lands to be included in the district meet the requirements of this section and either extend across county boundaries under one ownership or are contiguous to the land of owners in the adjoining county wishing to participate in the herd district. The county commissioner boards of each county desiring to participate in a joint herd district shall comply with all the provisions of this section dealing with single-county districts. A joint herd district shall be created only after approval by all county commissioner boards of participating counties as provided for single-county herd districts in this section. Joint herd districts may be dissolved in the same manner as single-county herd districts.
§ 81-4-304, MCA