It would be nice to have a way to associate multiple images with albums. But it would have to be done without compromising the simplicity and performance advantages of the current one image to one track relationship. Since multiple images can be saved in most file types, the most obvious approach is to support choosing which of multiple images to display. If that could be done, then it should also be possible to include commands to "flip" images (which would likely be a simple toggle of image 1 and image 2) and the option to display multiple images in a view (maybe just as a cover is now, but with a "back" beside it, if there is one).
I don't think there's any future in associating files with physical media packaging. I understand attachment to jewel cases and shiny disks (we have, after all, been brainwashed by billions of marketing dollars), but these things are going the way of the 8-track. I much prefer a more general approach, where different kinds of files (and not just images) can be associated in different ways. I saw great potential in the Notes feature as a more general tool that would allow any information to be associated with any Category (rather than files). That would have allowed multiple album art, reviews, log files, etc. to be associated with albums, and artist photos, bios, etc. to be associated with artists. The idea was this information would be displayed in a second pane—just as you've asked for. Sadly, JRiver showed no interest in the idea.