I practice in NY and most of clients are in NYS so I'm not entirely sure how to deal with this. 

A client of mine is in Princess Anne, MD and her spouse has income from DE. He pays tax on the DE income and receives a tax credit in Maryland against tax paid in DE. However they also have a local county tax against which they get no credit. There was a case a few years ago that I came across in researching this, Comptroller of the Treasury of Maryland v. Wynne , that I believe dealt with this issue - there the Supreme Court decided (long story short) that basically localities can't double tax people this way because it violates the commerce clause. 

However the taxable income flows automatically to the local tax form in the maryland return and the only way I can change it is to override it. So basically I'm wondering 1) is my assesment of this issue correct and 2) if I override and attempt to e-file it will MD reject it?

thank you for your help and any further material you might be able to refer me to.