Just noticed this thread. I'm of the opinion that collaborative composition is in some ways and in some situations more interesting than the "lonely genius" model. Since 1986 I've been involved in making electronic music in the duo FURT with Paul Obermayer, for example. There is indeed a significant proportion of spontaneous improvisation (and therefore ipso facto collaboration) in the way we work, but this is interwoven as necessary and appropriate with compositional planning, so that the end result can't really be described as one thing or the other, although different performances of the same piece are certainly recognisable as such. I find that the work-process here involves much less hesitation and uncertainty than "solo" composition, so that the constant feedback, mutual encouragement and criticism between Paul and myself speeds up the process enormously; it's also apparent to me that the music of this duo has developed a "personality" of its own which is somehow different from the sum of its parts (for those who might be interested, the process is discussed in more detail in the two interviews here: http://furtlogic.com/i.html).

There are other compositions I've worked on which have been mostly composed during the rehearsal process in collaboration with performers, an idea inspired largely by the work of my own principal composition teacher Peter Wiegold, and others (involving more complex electronic applications) which have been developed in collaboration with computer programmers (a practice which is common in institutes like IRCAM, where the creative input of the "assistant" is often grossly underestimated). Situations like these are more in the nature of "division of labour" where the different participants bring specific skills and ideas to the collaboration.

Aside from my own experiences there are plenty of others which I think are of interest: the case of Giancinto Scelsi, for example (see his thread in "20th century), or various experiments initiated by Karlheinz Stockhausen, from Carré (planned and structured by Stockhausen, worked out in detail by Cornelius Cardew) and Plus-Minus (a kind of "construction kit" from which quite different-sounding compositions can be realised) to Ensemble (where Stockhausen devised an overall scheme and some tutti episodes which enclosed and contextualised twelve pieces by twelve different composers, each for a single instrument).

All of these examples are quite different from the traditional relationship between composer and arranger/orchestrator, in which the "collaboration" is basically sequential rather than simultaneous.