I don't think my system is fast enough to do simultaneous burns. It takes lots of resources even to do one burn. Let me show it in another way. I tried before to do quality test simultaneously. Plextools sum8 test on my 716a and Nero quality test on my 1620a, simultaneously. On plextools I cannot see any "hickups" since there is no read speed shown during test. However in Nero I can see reading hickups. Look at the green line in the pic below. Usually there is no small spikes up and down on the green line (you can see other Nero quality posts).
I put both of my drives in the same IDE channel though, since I'm not interested in on the fly copy anyway. You might want to try to put the drives in different channel, or better in ATA controller for 716a (UDMA4) and play around.