I assembled 2 PDCs on a newly colonized planet, because the population was nagging about protection.
The 2 PDCs are identical, and each have room for a single batallion.
Now, I moved one garrison batallion into PDC 1, and that worked fine.
However, when I try to move the other garrison batallion into the other PDC, it does not work.
Is there a rule that only 1 PDC on a planet may be occupied by ground troops?