http://musicbrainz.org/release/3a324e54-360a-4aeb-86f8-16076f2bde4e/discids

1) Release was added as a one disc release, using an existing release with same track listing as the basis.
2) CD Lookup in Picard, Lookup manually
3) Select newly added release to attach the disc ID
4) http://musicbrainz.org/release/3a324e54-360a-4aeb-86f8-16076f2bde4e/discids now shows "Format:2xCD"

(In case it's relevant, edit to Add Release in 1) was still open when 2-4) were done.)