OK, that is much different from what we do up here in NYC/NY.
So if they calculated the county tax completely from scratch, instead of using a percentage of the Maryland tax, this would not be an issue because your out-of-state credit would be already ‘used up’ on your Maryland tax.
This is opposite of the NY/NYC commuter tax. That taxes people who do not live in the city for working in the city. This taxes income earned outside the state for the portion going to counties while rebating the state portion.