Your description is there is no Montana source income only Idaho source income from when you were residents and Idaho source income from when you were non-residents.
You will claim a credit for the Idaho tax paid on income taxed by Montana after you became a part-year resident.
Going forward you will claim a credit for the Idaho tax paid on the business income generated in Idaho that is taxed by Montana (100%).
Montana Form 2 Instructions Page 39 Col 1 - Credit for Taxes Paid to Another State
