Let us know the following information:
1. Which are the two distros you are trying to dual boot.
2. grub.cfg or grub.conf (depend on the distro you are using) output. Do the following from the distro you are able to boot in: cat /boot/grub.cfg or cat /boot/grub/grub.conf. Paste the output here so that we can have a look.
3. Also mention (if you remember) on which partition you have installed the the distro that you cannot see in the grub or at the boot selection screen.