This is an old thread but likely both states (MD and CO) taxed the income with a credit for taxes paid to another state on the CO state return.
MD rules say, for non-residents, the income is MD sourced if it is "Derived from a business, trade, profession or occupation carried on in Maryland."
CO, like all resident states with an income tax, taxes income received while a resident in CO. That sets up the double taxation.
The taxpayer should have filed a part-year return for both states and allocated the income first according to where they were when they received the income, but MD also gets to tax the non-resident MD source income received while in CO (the double taxed portion).
You can then claim a credit for taxes paid to another state on the CO resident return to mitigate the double taxation.
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