Here's the rule: a demoralized army (or force) loses additional steps after the battle due to cavalry pursuit.
Here's the situation: an Austrian army and a Prussian army are stacked together. A French army attacks them and wins the battle. Both armies retreat together but check for demoralization separately. The Austrian army becomes demoralized but the Prussian does not.
Here's my question: considering that the Austrians and Prussians are retreating together and that the Prussians are not demoralized, do the Austrians still take losses due to cavalry pursuit?
What has happened is that the two armies are a single force, not two different ones. To "aggregate" an army really is the same thing as integrating it.
An army group (even of allied armies) has an overall commander and make one roll for demoralization. This was covered in message #1472 on CSW:
The multiple armies force is a force and you compute the moral of the force based on the total of the steps in those armies. There’s only one morale test for the force; it it fails, every army is demoralized. The losses are shared between armies but at least half of the losses must come from steps which determine the moral value.