I just found that Nero would open twice, I have a sneaking suspicion that using two open instances together may be an instant coaster factory though?
Using two different programs might not be any better - If the writers share an IDE with each other or either source, waiting for bus availabilty could slaughter the throughput.
It would also be asking a lot for a single HD to provide 2x 22Mb/s source rate, while seeking between two different source files or groups - with independent source drives (they could share an IDE without impeding as badly) and independent writer IDE connections, then running two writer instances together MIGHT hold together - but it could still drop in a heap!