Put an HD and CDROM on each channel.
Sharing bandwidth between the HDDs will only really show a performance hit in benchmarks--you won't really feel it. That is not the case with readers and burners. I don't know why, but even now, they'll take a real hit with both on the same channel.
The overwhelming majority of newer IDE controllers will allow for different DMA modes on devices, so you won't get a performance hit to the HDDs from running the slower CDROM on it (this used to be an issue way back when). However, you may have a small hit, dunno if it will be noticeable, transferring from the CDROM to HDD on the same channel.
If you plan to be transferring on the same channel with my suggested layout, and don't even want to risk a performance hit (especially if it rears its head with burnproof--ugh, slow...), then get a controller card and cables, with only one device on each cable. Not an amazingly elegant solution, but it will be one sure to work.
Controller
If you're going to have four cables, either learn some cablegami to get them out of the way of the air flow in the case, or get some rounded cables (SVC usually has some decent ones at good prices).
