@barnee
For game play reasons, an Army Group is a single Combat Formation - and all rules that concerns an Army Group must be kept in order to avoid confusion.
In other words:
When an Army Group attacks, all units inside the Army Group must fight on Army Group terms.
An Army Group cannot interact with units outside the Army Group - which means that a Tactical Bomber unit cannot support a Tank unit that is included in the Army Group etc.
This also means, that only one of the two Armies that form the core of the Army Group can fight as an Army - whereas the units in the second Army in the Army Group must fight as individual units.
So in the same territory, a Panzer Army outside the Army Group cannot fight as a Panzer Army, since only 1 Army in each territory can obtain its fighting strength as an Army - and that Army is inside the Army Group.
If you want that Panzer Army to fight as an Army during your attack, you would have to “split up” the Army Group during this attack - then all units from the Army Group fight as individual units - and the Panzer Army can then fight as an Army during this attack.
Does this answer your question?
The only exception is when two or more Armies are defending Eastern United States (Washington), Germany (Berlin), United Kingdom (London) and Russia (Moscow) - in this case (Capital Defense Rule, see pg. 48, note 7) two Armies can both fight as an Army - whether they are part of an Army Group or not.