The Release Group style guide indicates that the following is not considered a "compilation":

"A release containing two albums and/or EPs." (1)

Musicbrainz defines a compilation as:

"a collection of recordings from various old sources (not necessarily released) combined together." (2)

A 2-on-1 release fits this definition precisely, and is only excluded from being a compilation because the style guide says so, and not for any particularly logical reason.

Furthermore, if we ignore the number of discs as being significant, there are a great many "M-on-N" releases, with all but the 2-on-1 variety already considered compilations without question.

Examples:

2-on-1 : http://musicbrainz.org/release-group/5155ae5b-bc96-3501-b99d-5cbe68d4dfdf
4-on-2 : http://musicbrainz.org/release-group/2337843b-fa02-377c-aec3-ccd8d3ee1742
5-on-4: http://musicbrainz.org/release-group/4479a656-9a69-3315-b3d2-4d0f030977a6
5-on-5 : http://musicbrainz.org/release-group/16cb2408-9b3f-42d1-9d22-db044b9e15f9

All the way up to releases like this:
lots-on-lots: http://musicbrainz.org/release/5965f781-45b7-4a7b-981b-1a94cd688fa0

The 2-on-1 is different only in the sense that it is a single disc, but it is still a collection of previously released recordings, and not a new release, and therefore it should be a compilation.

The proposal is to remove the line "A release containing two albums and/or EPs." from the official style guide and associated documents.

(1) https://musicbrainz.org/doc/Style/Release_Group#Secondary_types
(2) http://musicbrainz.org/doc/Release_Group/Type