Tried to do some co-op with the OXMmers tonight to unlock some of those juicy two-player achievements; there were about eight profiles/gamertags between two memory units and a hard drive. End result: 70 points, two corrupted save files (including the total destruction of one that had all the tracks unlocked) and a third player who got Achievements for things he never did (but one of the other players had).
In both corrupted save instances, it happened after an autosave to the same device that stored the currently in-use save file — I’m guessing Player A’s save file was in use and Player B’s profile attempted to save to Player A’s space, and then it all went to hell. The new save appeared as “Unknown Player” instead of the proper profile name, and the original save file was no longer accessible.
I think RedOctane has some patching to do. Proceed with caution, yo.