Ah, okay, so this is something more specific. My next guess is that you're running into the issue that Diamondus Ultimate and Jungle Ultimate have several files with the same filenames but different versions, particularly DD-Anims.j2a. Try extracting all the files from one of the zips, overwriting any files already in your JJ2 folder that have the same filenames... that should force your game to use the versions of the files that each level expects.

Because Jungle Ultimate was released later, I hope that it accounts for this issue, and hope that installing Jungle Ultimate after Diamondus Ultimate (overwriting everything) makes both of them work at once, but I didn't make these files, so I don't know for sure.