Em vez de complicar as coisas com duas imagens, você pode dizer ao GNU ddrescue para pular as partes lentas e voltar para elas mais tarde.
O sinalizador que permite fazer isso é --min-read-bytes=
.
--min-read-rate=bytes
Minimum read rate of good non-tried areas, in bytes per second. If the read rate falls below this value, ddrescue will skip ahead a variable amount depending on rate and error histories. The skipped blocks are tried in additional passes (before trimming). If bytes is 0 (auto), the minimum read rate is recalculated every second as (average_rate / 10).
Se você insistir em criar várias imagens, o manual também tem um exemplo de como combiná-las:
Example 4: Merge the partially recovered images of 3 identical DVDs using their mapfiles as domain mapfiles.
ddrescue -m mapfile1 dvdimage1 dvdimage mapfile ddrescue -m mapfile2 dvdimage2 dvdimage mapfile ddrescue -m mapfile3 dvdimage3 dvdimage mapfile (if bad-sector size is zero, dvdimage now contains a complete image of the DVD and you can write it to a blank DVD)