It's much better NOT to use Nero's multiple recorder option (available only on retail versions) unless you have identical drives. With 2 different drives, one will be in a constant state of buffer under-run due to the fact that they do not burn at the same speeds. This is asking for problems.
You can always burn from 2 separate instances of Nero, but this will place a huge burdon on your HD, and most HD's will not exceed 4x speed on 2 concurrent burns. (again, constant under-runs) So, it may be faster to burn each disc alone. Sometimes, depending on the HD, you can get away with staggering 2 8x burns, so that the second one starts in the middle of the 1st. This makes the total data rate lower, as both drives will not be at 8x at the same time.