I burn 2 discs at once all the time using 2 copies of toast.
I'm sure the only speed limit would be your hard drive (transferring data to the burners), not the IDE bus... I'm kind of amazed anyone even mentions it, that's just silly.
I use a RAID-0 for my hard drives and have no problems. I don't know about anybody else though.
I get the feeling sometimes that the hard drive is slowing, but if the burning is all that's going on, it works fine.
Edit:
Btw, there's a lot of pictures of stuff you can do with drives in the mac pro here:
http://www.tenthousandpercent.com/?section=other