The API reference is not entirely clear, but it does not appear that you can create new mirror columns:
You can read the mirror column via the API, but you cannot filter, update, or clear it.
This seems rather unfortunate if true, and the reference needs to be revised to make that clear. The API currently returns the following error when trying to add a new mirror column to a board that has an existing connected board with other mirror columns already present:
errors: {
error_message: 'This column type is not supported yet in the API',
error_code: 'InvalidColumnTypeException',
error_data: { column_id: null, actual_type: 'lookup' },
status_code: 200,
...
}
At the very least it would be very helpful to be able to add additional mirror columns to a board if a link to the connect boards is already present.