I ripped a two CD album via my streamer with an internal SSD drive. Each CD was ripped to a different subdirectory.
I then added first the directory with the first CD and then from the second to have them in the right order, selected all, Discogs as the source and find the album. Unfortunately, when tagging, for some reason, MP3tag did not follow the order of the files as imported to tag and mixed them... I ended having to tag each CD separately.
In the discogs search result dialogue, you can drag the files in the right list so that they match the entries in the left list.
In general, the data found in web sources depends on how it is represented by the web source - MP3tag has no influence on the way that web source does and in which quality the data appears.