lets say you have a fedora installed on your first partition and of course it has /boot in it.
and let say we will install another OS in your 2nd partition which you will just have to copy the vmlinuz, and initrd.gz to boot the 2nd OS.
adding a list in the menu.lst in the /boot of the first partition is required then.
I'm not completely sure what you're saying, I apologize. But if you're saying what I think, and that basically you have to copy the 2nd OS's kernel (vmlinuz & possibly initrd-img) to the /boot directory/partition of the first OS, that's wrong.
You can have a separate /boot directory for every OS (and in fact, for Linux you have to I believe). You just set up another entry in menu.lst specifying the new path of the kernel & ramdisk. I.e.: if the other OS is installed on sda2, and there's no ramdisk/initrd-img, the entry will look like:
Code:
title Second OS
root (hd0,1)
kernel /boot/vmlinuz-3.42.3
If I mis-read your question I sorry, please let me know.