The separated ones (which are merged and treated as one on my hard disk) simply come forth in MC as numbered albums, for ex.: Requiem (1), Requiem (2). MC seems to treat them in every way as separate albums. Is there a way to merge them?
Just change the name of both Requiem (1) and Requiem (2) to "Requiem". You can use the slow double click on the Album field to make the field editable, or click on it and press the F2, or right click on the Album name and select "Rename".
I suggest that you use the "Albums" View in MC to do this, then search for "Requiem" using the search box in the top right corner of MC Standard View. You will get a view something like the first attached image, once you display the bottom pane. Before you merge the two discs into one Album, it is a good idea to display the [Disc #] column (right click any column, click on "Disc #", click anywhere outside the list of fields) and add in the correct disc number for all the tracks, as highlighted by the
Blue ellipses in the image. This is a good time to check and if necessary add in the Track numbers as well, as highlighted by the
Green ellipses.
Note that in my simple example, I have a two disc set of "The Essential Neil Diamond", but the discs are named "The Essential Neil Diamond (1 of 2)" and "The Essential Neil Diamond (2 of 2)" as highlighted by the
Red ellipses. Note also the "Album" column shows the two names.
All I have to do to merge the two discs into one Album is to select all the tracks shown, then edit the [Album] column so that they all have the same name, "The Essential Neil Diamond". Done.
See the result in the second attached image. That is using the default sort of [Artist], [Album], [Disc #], [Track #].
The Panes display you were using will also show the Album correctly.