From how I understand Release Groups, It would make sense to have these in the same one, however the releases should of course stay together and not have their disks renumbered. Is there a recommended way of doing this?
I’m not a classical style guide expert, but both releases’ artist credits seem to have some issues with not matching what’s on the uploaded cover images. The 1989 release doesn’t abbreviate the composer’s name the same way as the cover, doesn’t credit the orchestra and chorus, and credits additional performers who aren’t listed on the cover. The 2004 release is much closer but credits “Chorus and Orchestra of Netherlands Opera” instead of “Chorus & Orchestra of Netherlands Opera”.
I agree with Derat. What you want to do is merge the release groups, not the releases. That way the individual releases will remain intact.
I would do the merge and worry about the credits after the merge has been applied, in case any of the edits might cause the system to see a conflict, and cause the merge to fail.