I'm not sure what happened here, but take a look at this release: http://musicbrainz.org/release/d5dc2370-5e38-4dda-95c1-56b4e3a6ec64

1) Actually, I opened 4 tabs: each per a track
2) Then I clicked on "Use in relationship" in first 2 tracks
3) Next I changed direction on relationship and added the "remix of" one
4) Then I repeated steps 2-3 for last two tabs/tracks

And then I refreshed the page with release, I noticed that despite it added "remix of/remixes" field to the release, they're still blank, though the relationships are present in the tracks: http://musicbrainz.org/recording/1d7451d0-62b5-4c86-902f-dc61709195b1
http://musicbrainz.org/recording/dd36870c-f8ba-4f27-b288-e1ff03ded88b