I have found that if you try to dual-boot Linux with Linux, the second flavor that you install overwrites the existing boot loader and the first distro seems to vanish. To solve this problem, don't install the boot loader of the second distro. You should be able to find an option for this as you go through the install process.
I know what you're thinking -- if I don't install the boot loader, how will I boot the system?
Let's assume that the bootloader is on the first hard drive (hda) from your original distro. You need to copy certain files from the /boot directory of the hdb drive to the /boot directory of the hda drive. To do this, you must first mount the hdb drive while logged in as the root user. Since this is only a temporary mount point, I just use the floppy directory:
Some of the newer distros now use the /media directory for mount points, so if the above command doesn't work, try:
mount /dev/hdb1 /media/floppy
You should see an icon appear on the desktop that looks like a diskette. Enter that drive, and
cd /mnt/floppy/boot or
There are three files you need to copy to the /boot directory of the first hard drive (hda). The first is System.map, which will look something like System.map-2.6.9-1.6_FC2 (which is the System.map for the 2.6.9.-1.6 kernel from Fedora Core 2). The other two files will have identical numeric suffixes, after prefixes of vmlinuz and initrd. Once you locate these three files, copy them to the /boot directory of the first hard drive (hda). When copying the initrd file, make sure to copy it exactly as you see it, including the .img extension that will be someplace in the name of the file.
cd /boot to change to the /boot directory of your first hard drive (hda). You should see the three new files added to the directory.
Now there's only one step left -- add a stanza to GRUB, the boot loader file. Change to the /boot/grub directory and open the file menu.lst in your favorite text editor. Scroll down a few lines and add the new stanza under the one for your existing distro, skipping a line between them. It should look something like this:
title "whatever you want to call your new distro"
kernel/boot/vmlinuz"Your Kernel Number" ro root=LABEL=/
initrd/boot/initrd"Your Kernel Number".img
Note that this is a case-sensitive and space-sensitive document. That means that root=label is different from root=LABEL, and there is no space after the "=".
If you've done things right, you can now reboot your machine and you will see your new entry in the boot menu. Select it and you will boot into your new distro.