Your CD will need:
1. the initrd and kernel images of every distro you want to boot from HD
2. a startup script (in every initrd) which mounts the usb storage drivers, enumerates the HDs, and checks each HD to see if that is the one with the appropriate 'root' filesystem
Now if the script in  finds the right HD, then it can mount the HD partition as 'root'. If it doesn't find the HD, it needs to leave 'initrd' as root and at least drop you into a system that has enough functionality available for you to poke around and find out what's going on.
As you can probably imagine, this can be a nuisance at times because changing the kernel on each HD means you need to rebuild the initrd and CD.
The CD can never boot directly to a USB HD if the BIOS does not support USB booting. The reason is that the BIOS needs to provide the initial access to load the kernel and initrd (if you use initrd). So no BIOS support = effectively no HD to boot from. To be able to access the USB HD, you need an initialized kernel with the proper drivers booted - that is why your CD needs those initrd images - from there you mount the USB HD and finally 'chroot' to the final (r/w) root system. There are many good things about this approach anyway - for example, the system running under initrd can run fsck on the final root partition before mounting it.
By the way, there is no need for multiple USB HDs - you can have as many distros as you want on the one HD (even share directories such as 'home' if you want) - this really has insignificant effect on how you prepare your boot CD.
If that all sounds like ancient Arameic (or Sanskrit, Minoan, etc) then the answer is:
Unfortunately, the best preparation for what you want to do is to go build your own system from scratch (see http://www.linuxfromscratch.org
). Then take apart the *.iso image from a suitable distribution (Debian is OK for this) and study the entire bootup process from the "init" program on to the mounting of the final filesystem. When you've done all that, then it's pretty easy to see how to do what you want. Unless you have a lot of free time and really love doing this stuff, give yourself about 3 months to get up to the level of expertise needed to do what you want.
No 'quick' answers for this one; this is kind of like asking 'how do we put someone on the moon and get them back alive?'