When I recently wrote about the new Decca Ravel: The Complete Edition release on my Examiner.com national site, I made it a point to observe that the use of the adjective "complete" was a bit of a stretch, using Grove Music Online to track down a few original works that were missing. There was also a bit of variability over how Ravel was represented as an arranger. The orchestration of Modest Mussorgsky’s Pictures at an Exhibition was not included, which I felt was totally understandable. On the other hand I was glad to see that the collection included Ravel's two-piano arrangement of Claude Debussy's three orchestra nocturnes (which is now my second recording of this impressively successful effort).
However, while I do not mind the absence of the Mussorgsky arrangement (partly because I could not begin to enumerate the number of times this has been recorded), I regret that the Decca collection did not account for the fact that Ravel's relationship with Debussy cut both ways. Not only did he distill some of that composer's richest orchestral writing down to two pianos, but also he orchestrated a few that same composer's piano pieces. The Naxos Complete Orchestral Works Debussy includes Ravel's orchestrations of the sarabande movement from Pour le piano and the short "Danse" piano solo (along with other orchestrations by composers contemporary to both Debussy, such as André Caplet, and the present day, such as Robin Holloway).
Off the top of my head, I cannot think of another composer who took this "bidirectional" approach to arrangement with anyone, let alone a colleague as close as Debussy was for Ravel.