Ok, I see ... I probably would not try a splitter ... I'd probably get rid of either the CD or DAT drive.
I can't speak from experience or from documented knowledge, but there would probably not be any issue. I have used a number of technically-unsupported optical drives in some of the 6G/8G servers (took the DVD drive from an old HP laptop, which worked). I doubt there are any validated DVD drives for the 2400, so you might have no choice other than to "try" one. But, maybe, just maybe, there is someone with one of these machines that might know this first-hand ... not sure how long you want to wait ;)