Like the Title says it, I've have wondered this for a few time now and it would greatly help.
Its is possible, all u need to do is have one as the Master (HDD) and the other as slave (CDROM) and make sure that in your bios, u have the ide as the primary boot, plus make sure u have IDE running as NATIVE, not LEGACY, u will see that option somewhere in the boot options (If i remember correctly).
As mentioned above it is possible, however unadvisable as the speed of the cd-rom will drag the speed of the ide bus down, thus slowing down the hdd.
When possibly always use separate ide controllers for hdd and rom drives.
Yes it is possible to do, but it will never work well, and somethings, such as copying files to a cd, or duplicating discs, will never work properly... and k.Jacko has an excellent point with the operating speed.
Cannot think of why you would need to unless the EIDE 2 socket is bad. If so, better to use a USB external optical drive.
MYTHS... Stop perpetuating them!
Having both a CD-ROM (or comparably slow IDE device) connected to the same channel as a hard drive does NOT slow the HDD down.
With older PIO drives (circa early 90's and prior) the CPU was in charge of handling all of the requests made by hard drives and CD-ROMs. This required drives to 'share' cycles and chew up CPU time. I could see how this conceivably would slow devices down sharing the same channel, as they have to take turns. I'm not aware of the real-world results though.
These days, hard drives and CD-ROMs are DMA, which is 'direct memory access'. These devices have their own 'processors' that handle requests directly to/from your memory. They do so independently and therefore, do not share cycles, which in turn means no theoretical loss in performance.
This has been a public service announcment for the good of humanity. Thank you.