I do not completely understand the issue.
Suppose you pay $2000 in another state, and your Maryland tax is $3000. You deduct the $2000, and pay Maryland $1000. If your county tax is $500, why should you be able to use the credit again? You’ve already used it against you Maryland tax.
In NY, city income tax is administered on the state form. You calculate your total state and city tax, and then applty credits against that.
In MD, the county tax is a percentage of the Maryland state tax, BEFORE you apply the tax credit for out of state payments. So while MD (sate), does not tax you on out of state earnings, the counties do.