2 step suggestion:
Scratching is an indication of having a reaction to something. The yards, lawn areas you walk by - have they been treated with something? Is your own yard treated? Look at their food, chemicals used in the house.
Look at their skin and coat. Irritation, discoloration. Good time of the year for fleas as well.
Get a trainer involved for the walking issues. I would suggest also walking them separately until you have a handle on it. That way you can concentrate on the training aspect of the walk with the 2nd GSD.