Chainloading an operating system allows grub to boot an opearating system's boot loader. This is commonly used to boot Windows for instance.
Suppose main grub installed on /dev/sda1 and the second os is installed on /dev/sda2
in /boot/grub/grub.conf or /boot/grub/menu.lst (which is grub configuration file)
add as following
title Linux 2 on /dev/sda2
# or use rootnoverify (hd0,1)
#rootnoverify is same as root but don't attempt to mount the partition.