Good news is that most systems will support up to 4 IDE devices. You already have 2 ( HDD and CDROM) and you are adding 2 more (HDD2 and CDROM2)
The biggest issue you have to deal with is cable length. What matters the most in adding drives is what will the cables actually reach to and plug into. The most important thing you need to remember is that you place your Operating System Drive on the Primary IDE Master Setting.
This being said it is important to inform you that each IDE channel has a master and a slave. For each drive being added you must make sure it is not set to conflict with the other drive on the IDE chain. If the existing drive is set for slave, then the new drive installed on the same IDE channel must be in the master setting.
Now, as to the optimum way to install your drives for the fastest use I've heard many schools of thought. Really I believe it comes down to your IDE controllers and the cables being used. If you only have IDE controllers that support ATA 66 max then you can put the devices any which way you choose barring the Operating system drive.
Now I've heard that ATA100/133 should be used for your hard drives only, as placing a cdrom (ATA 33 speeds only!) will slow the bus down to the slowest speeds- not sure if thats entirely acurate or not.
Some would also argue that by placing both HDD on one cable, and cdrom's on another will slow your system down if you are doing a cd to cd direct copy, or a hdd to hdd write. This is due to both IDE devices accessing the same IDE buss to read and write.
So if you only have ATA 33 /66 controllers, then mix up the devices, since the cdroms will not slow any speeds down. If you have a ata 100/133 controller then you might want to put both IDe drives onto that controller and the cdrom's to the other.
The other option is to buy a PCI IDE controller for your HDd's and then put the Cdroms on the MTB IDE.
WEEEHHHHHHHHh! way too long of a post! SOrry bout that guys :-)