I have the same problem to get multiple artwork.
I set in Preference -> Local Correct -> Artwork Correct -> Maximum artwork images that can be added to song = 2.
But I get only the front cover, however MusicBrainz has the back cover as the second image too (linked to archive.org).
Currently we only try and find the front cover when getting artwork from online resources. This is because this is generally waht people want, and was the only piece of artwork we had some coverage of.
Now that MusicBrainz has started adding support for this more directly via the Cover Art Archive I expect to implement support for other artwork types at some point.