Mac the Knife,
The new Dual IDE will run drives at different speeds but it has to be on a motherboard that supports it and it has to be setup properly to work.
I have found very few computers setup properly. For best performance you still want one drive per ribbon. (That way one drive does not need to wait for the other drive to finish with the IDE controller.)
A good test is to see if your computer is running the IDE with independent device timing is to use a benchmarking program like:
With SATA becoming mainstream now, the Master/Slave ATA Speeds is going to become a non-issue.
Take Care,
Richard