I don't have 2 burners to test this with, but here's a thought:
I know that when any burning program writes the Lead In, it essentially takes over the IDE channel, forcing other operations to wait until the burner is done with that portion.
I thought I remembered reading somewhere (on these forums... I just don't feel like searching at the moment) that if you turn on "Disk-at-Once/96" mode when selecting the burn speed, etc., it will allow the two burners to run much more happily, as the machine just sends a pure data stream to the burner, the CPU having done most of the other calculations. Of course, this requires a relatively new CPU (maybe 1.5GHz or higher?).
Good luck and let me know.