Here's the problem in Ohio: Unlike the federal tax system, Ohio does not have separate tax rates based on your filing status. The 2nd spouse's income is added to the first spouse's income and that total income is pushed into a higher tax bracket. So, a single person and a married couple making the same income are each taxed at the same marginal rate. Ohio tries to compensate for this with a Joint Filing Credit, but it is limited and doesn't always make up for the higher rates. The Joint Filing Credit also requires qualifying income, so if the spouse does not work (or have retirement income other than social security), the couple does not usually even get the credit.
Each Spouse must have at least $500 of qualifying income to get the credit. The income must be "qualifying" income. Typically only earned income (e.g. wages) and retirement income are qualifying income. Interest, dividends, rental income do not count.
Then, any Ohio schedule A deductions (like excess medical expenses) or Federal adjustments (the most common being an IRA deduction) must be subtracted from the qualifying income. If either spouse has less than $500 net, you will not qualify for the Joint filing credit. Take a look at the joint filing credit allocation worksheet (abbreviated Joint Alloc Wks on list of forms) that TT does, it should show these adjustments and the net qualifying income for each spouse.
