When installing fedora, you'll have an option on where to install grub. Just install grub in the boot partition of the fedora installation, or the root partition if you don't have a separate /boot partition. That way, the existing grub menu in the MBR wont be overwritten.
You'll probably have to edit mint's grub menu to allow you to boot into fedora. Also, fedora may automatically add mint as a boot option (you can edit this during installation), so you might want to edit the fedora grub menu as well so you don't have to go through 2 grub menus when booting fedora.