When two or more artists work on a track, there is no formal way of tagging the track with each artist separately.
Banshee should come up with a method to make this possible. I have a couple of ideas.

First, for the interface, an 'Add Artist' interface should be added in the Edit Track Information dialog.

Second, I believe that the current metadata tagging systems work based on the existing standards for music file metadata integration. And of all I've read about them so far, tagging formats like ID3 have - I fear - a very limited and awkward capacity to implement this multiple artist feature. As a workaround, Banshee could store the multiple artists as a comment in the track, and sort them accordingly when loaded to the library. 

The idea behind this is to
1. Have an approach for specifying multiple artists in one track with an official method - at least at Banshee level.
2. Let the user, upon the search of an individual artist, obtain within the results tracks that have been product of the collaboration with other artists, and have the possibility to perform further actions involving the other collaborating artists.

As opposed to having this feature, users have to chose a way to record in the metadata the collaborators of the track in their own way. 
For example, if I have a track produced by artists A and B, the only thing that comes to my mind is to make the track's artist "A, B". But now, if I were to search all the tracks by B, and I searched for "artis==B" (to avoid results from other artists containing parts of B's name), B's song in collaboration with A will not appear within my results.