If what you're asking is how you can get grub to look at a different OS partition for the /boot/grub then you have to run grub as root. From a terminal prompt type:
It should return something like:
assuming the drive and partition of your other distro was (hd1,6) you would run:
then, if your boot drive was (hd0):
You might also want to setup grub on the MBR of your other drive (if you have more than one) as well. Finally, type:
That should get you booting from the proper grub. You'll, of course, want to add your linux mint entry to the menu.lst file on your other OS partition. but that should do it.