I ran a simple test:
I played a CD on one, and burned a CD on the other, at the same time. Both worked fine. Now, that could be because of the buffers involved with the burner, and the fact that when reading a music CD the CD is not accessed all the time, or it could just be that BUS mastering works as supposed to?
I have moved them onto different controllers, but still wonder if that is actually better than running them off one controller.