The answer depends on where you actually perform the work, and what states are involved.
If you simply have clients from the neighboring state, but you perform your work in your residential state, then your income would only be taxed by the state of residence.
If you actually travel to the neighboring state to perform the work for that client, then that state may tax the income earned while working in that state. You would need to investigate that further with the tax laws of the neighboring state. There may be an income threshold to consider before a return is required, or that state may not have income tax at all.
@bjpehlman