You can have multiple instances of grub installed. This is handy if you have one distro you normally use but want to have another distro to play with.
Here's an example:
Install Ubuntu to hda1, grub on mbr, hda2 will be a swap partition. After that install your second distro to hda3. Be careful to install grub of this second distro to the boot sector of hda3 and not to mbr. Add entry to menul.lst that chainloads the second grub. Also reset the default time before boot from 3 sec to 7 or 8 sec.
title Alternative GRUB
root (hd0,2)
makeactive
chainloader +1
This allows you to freely repace the 2nd distro with another one and you don't have to change the menu.lst in Ubuntu. Just wipe out hda3 and install the distro of your choice.
Also, if you upgrade the kernel of your secondary distro, it will likely update menu.lst to contain this new kernel, which becomes automatically available to you. If you use only one instance of grub you'd have to manually add new kernals of your secondary distro menu.lst in Ubuntu.
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We have this working at the computer club and I have it working at home.
You can have up to 3 distros, using hda1, hda2, and hda3. Save space for swap (and home if you want to use the same files with multiple OSs) and put the remaining space into an extended partition and then create swap (hda5) and maybe home (hda6)