When you try to split and artist it provides you with the current name which will be something like "A & B". You can delete the "& B" part, hit lookup, find the right name and select it but despite it looking like you've linked to artist A, the edit actually acts as if you were creating a track by both "A & B" & B but where "A & B" have been titled "A" just for this release (in the same way some artist release tracks under different titles).

It looks like it works on the surface but anytime you click on "A" it'll take you to the page for "A & B"

Hope that makes sense, it's a tricky one to describe.

I've tried to split "Helmet & House of Pain" and this happened every time, at the release group level, release, track, recording.

If you look at this list of recordings by "Helmet & House of Pain" you can see some are by the combined entity, and some by the separate groups. The only reason both types are listed here is because "Helmet" on it's own is actually an alias for the collaboration.

http://test.musicbrainz.org/artist/5b08d7ce-faad-4ff2-abed-8bbe14b1c1a3/recordings