Washington and Baltimore share a large portion of Virginia and I think parts of North Carolina as their "Broadcast Market". I can't find a good map to show the territory, but Mama Googles says the teams share DC, Maryland,Virginia, Delaware, Central and Eastern North Carolina, South Central Pennsylvania (surprising because you'd think there would be a dividing line between Pittsburgh and Philadelphia) and West Virginia.
The Nationals joined the MLB.com group this year so I'm not sure this still holds. They and Baltimore shared MSN as provider for their games