There are several reasons someone may have to file more than one state tax return. If you moved from one state to another mid-year, transferred to work in a different state for part of the year, or you and your spouse worked in different states and you are filing jointly, you may need to file part-year or non-resident returns in multiple states.
If you have earned income in more than one state, on your resident state ("tax home") return you must include all of your income for the year - even that earned out-of-state - and then you must also file a non-resident return in your work state(s). That means, at times, those working in multiple states may have a double tax burden on their out-of-state income for the year. If this is the case, you may be allowed a tax credit for the duplicate income taxes on your home state return.
Occasionally, in areas where many people work across state lines from where they live (e.g., New York and New Jersey), the states may have reciprocal agreements, meaning the work state agrees to exempt the out-of-state worker from income taxes. In states with reciprocal agreements, a taxpayer must file an exemption form with their out-of-state employer so as not to have taxes withheld. In such cases, you would claim the full amount of your income from both states on your home state return.
If you moved between states during the year, and your move was permanent, a part-year state return is required for each state you lived in. In this case, you only have to include the amount of income you earned in each state on their respective returns.
If you and your spouse worked in different states, but you want to file your state tax returns jointly, you may do so by filing jointly (under both spouses' names) in both states, but including only the income earned in each state by the one spouse.