I answered my own question:
Your Montana filing status must now be the same as your Federal filing status.
- Prior to the 2024 filing year, many couples were able to reduce their tax obligation (or increase their refund) by filing “Married Filing Separately on the Same Form”. This filing status no longer exists in Montana. If you file “Married Filing Jointly” on the Federal return, you must file with the same status on your Montana return.
- Source: Understanding Changes in Your 2024 Montana Tax Return - Rocky Mountain Accounting
Filing Statuses
Under the new law, taxpayers must use the same filing status used on the federal tax return. These filing statuses include married filing jointly, qualifying surviving spouse, head of household, single, and married filing separately. When filing the Montana Form 2, each of these filing statuses is considered the same as the federal filing status. Married taxpayers can no longer file separately from their spouses unless they are filing separately for federal purposes. Capital losses, passive losses, and excess business losses will no longer be allocated by spouse and will be treated the same as they are for federal purposes.
Source: Montana Tax Simplification Resource Hub - Montana Department of Revenue
