There may be some other way to do it, but this is how I do it.
I install the new Linux distro "the normal way" without caring for grub at all.
Then I boot into the newly installed distro and repair grub
http://www.linuxmint.com/wiki/index.php ... _your_grub
The final stage is to copy the important line(s) from the new menu.lst to the one in Mint you want to use so you can boot the new one too.
And I've done this a few times now