You didn't move and you both were residents of MI for the entire year. File a resident return for MI and a non-resident return for his SD income.
When both of you complete the My Info Personal Interview:
-
Select Michigan as your state of residence;
-
Answer "No" when asked if you "Lived in another state in 2020?" That question only applies if you changed your state of residence in 2020.
-
When asked if you "Earned money in another state?", answer "Yes" (because your husband works in SD 20 days of the month).
Both MI and SD will tax your husband's SD income - MI because he is a resident and SD because he earned it there. MI, however, will give him a credit (against MI tax) for whatever tax he has to pay SD on the same income. This is how states avoid "double-taxation".
Prepare the SD return first - this is how MI will be able to know how much of a tax credit he will get against his MI taxes on the SD income.
