trying to figure which is which if they're both set the same is even worse. i'd put one a master and one a slave. then, at a glance, you can see which one is which. if that's too hard, then get sata drives.
some drives have to be set as master/single if there is no slave device, and when using CS, not all cables support that function. Almost any cable you buy today will have the notch in it to support CS, but not every company does that.
Also, never put an optical drive and a hard drive on the same chain. It slows down your hard drive.