Evidently that IS how it works, once the Read position counter (in Nero) for one of the drives reaches 100% the drives internal buffer empties and Burnproof kicks in, clearly indicating that one of the drives have gotten more than 80 Megabytes ahead of the other. Also the "IO bytes read" column on the "Processes" tab of the TaskManager clearly indicates that Nero has only read the source once. The idea is fine, it's just that Nero is a piece of junk, and 80 Megabytes of buffer is insufficient for my 2 drives.
Actually that's unlikely, The slowest drive would just get the files from the system cache, which unlike Nero doesn't have a 80 Megabyte limit.