Reason for double acylation of ferrocene only occurring with certain reagent combinations
One factor:  since ferrocene is covalently bonded, the rings "communicate" with each other via the molecular orbitals passing through the iron atom.  Thus the acetyl group placed on one ring not only deactivates that ring, but to some extent deactivates the second ring as well.  The second ring would still be attacked preferentially in the second stage but may require a stronger electrophile, which acetyl chloride is compared with acetic anhydride.