i have an asus cuv4x-e motherboard running a p3 1ghz and 640ram. I currently have two 80gb wester digital hd in ide 1, and a btc dvd-rom on ide 2 as master. I have a pci ultra ata 133 ide controler card with 2 lite-on ldw-411s, both on master(one in each ide).
With this setup i hAVE no troubles what so ever in burning simultaneously, and i always burn at 4x!! I use nero v6.xxxxx to do both burn at once, and it only consumes from 2-4% of processor speed when burning both at a time, so i can easily add two more to the equation!!!
LIttle thing to take into mind is that both dvdrw are using one ide each(master), both have same firmware FSOH, my controler card is pci, and always use same media at same time!! And ive been doing this for well over 6 months now with no problems at all!!
ps: my friend is also running two of the same dvdrw as me with same firmware on a HP computer without a controlercard(one master, one slave) and he has no problems either using nero v6.xxxx
oh and by the way Burning with alcohol and even dvddecrypter!!! uses more processor power than using NERO v6.xxx during the burning process, you can even check to see!!!