Mont. Code Ann. § 61-3-723

The right to the privilege and benefits of proportional registration of fleet motor vehicles, trailers, semitrailers, or pole trailers extended by 61-3-711 through 61-3-733 or by any contract, agreement, arrangement, or declaration made under the authority of 61-3-711 through 61-3-733 is subject to the condition that each fleet motor vehicle, trailer, semitrailer, or pole trailer proportionally registered under the authority of 61-3-711 through 61-3-733 must also be proportionally or otherwise properly registered in at least one other jurisdiction during the period for which it is proportionally registered in this state.