This feature would certainly be useful, but in many cases you might have to decide what to count as a collaboration. Liszt did many piano transcriptions of other composers' works, eg Schubert songs. I have checked a few entries and note that I not have been consistent here. Sometimes I have made up a combined composer entry, such as "Liszt-Schubert". In most cases I have just filed them as by Liszt. I don't think many people would regard Beethoven's famous "Diabelli Variations" as a collaboration with Diabelli. What about cases like Ravel's famous orchestration of Mussorgsky's "Pictures at an Exhibition"? Many other composers have also done version of that work.

You might decide to limit it to cases of composers actually working together on a composition. These are pretty rare. A fairly well-known example is the FAE Sonata, a four movement work with a different composer for each movement.

[url]http://en.wikipedia.org/wiki/F-A-E_Sonata[/url]

I have a very attractive CD, labelled "Collective Works by Russian Composers". One work is "Variations on a Russian Theme" with variations contributed by ten composers! You can read about it here:

[url]http://www.independent.co.uk/arts-entertainment/music/replay-1592495.html[/url]