The income must be "qualifying" income. Typically only earned income (e.g. wages), unemployment and retirement income are qualifying income. Interest, dividends, capital gains and 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 allocation form that TT does, it should show these adjustments.
In the "forms mode", look at the Joint Filing Credit Allocation Worksheet.
This is where your net qualifying income is calculated. It shows on line 4. If either of you is less than $500 on line 4, you do not qualify. Then, you can look at adjusting you allocations, where allowed, to get the credit . A good example is to forgo taking some of your medical deduction (or shifting some of the medical deduction to the other spouse), because the Joint Filing Credit is a bigger tax break than the medical deduction.
