Idaho's UM statute says nothing about stacking, so your policy wording decides
Idaho's rule is in Idaho Code 41-2502, which requires UM and UIM coverage unless a named insured rejects it, and the limits are set by 49-117. The tort deadline is in 5-219.
Read how stacked and single-policy coverage differ in Idaho, check what the statute says and leaves open, or see how UM and UIM work in Idaho.