Monoacetylferrocene undergoes a 2nd friedyl crafts reaction. Three products could be produced. What are they?

Well you will get a large yield of mono-acetyl but diacetylferrocene is harder to make.

The isomers you will get will be mono, which is easy just on one of the cyclopentadiene ligands.

Then you can get a diacetyl where they are both on the same cyclopentadienyl and two where they are on the opposite cyclopentadienyl rings.

The second and third are only slightly different because if you look at Ferrocene you will see that the cyclopentadienyl ligands are both staggered relative to each other. That means if both acetyl groups are physically on the same side of the complex, they differ from the other isomer where they are on opposite rings AND also on opposite sides of the molecule. This because if you imagine that the rings were to rotate around so they were totally eclipsed the rings would be eclipsed as well so making them different from the isomer where they are on the same side but with staggered rings.

You should draw it out and check it for yourself. Once you see the arrangement you will know what I'm talking about.