The aim of this tutorial is to create a Multiboot Linux Mint DVD.
The DVD will contain 32-bit and 64-bit editions of Linux Mint. For this tutorial I've selected Cinnamon flavor :)

Step 1
------------
Download the following ISOs from Mint's website:

- linuxmint-13-cinnamon-dvd-32bit.iso
- linuxmint-13-cinnamon-dvd-64bit.iso

Create the following directory structure:

$ mkdir -p mint-multiboot/boot/{grub,iso}

Now, move the two ISOs into the 'iso' folder (i.e boot/iso).

Step 2
------------
Create a file named grub.cfg in boot/grub folder and insert the following

##############################################################################
menuentry "Start Linux Mint 32-bit" {
    set gfxpayload=keep
    loopback iso /boot/iso/linuxmint-13-cinnamon-dvd-32bit.iso
    linux    (iso)/casper/vmlinuz  file=/cdrom/preseed/mint.seed boot=casper iso-scan/filename=/boot/iso/linuxmint-13-cinnamon-dvd-32bit.iso quiet splash --
    initrd    (iso)/casper/initrd.lz
}
menuentry "Start Linux Mint 32-bit (compatibility mode)" {
    loopback iso /boot/iso/linuxmint-13-cinnamon-dvd-32bit.iso
    linux    (iso)/casper/vmlinuz  file=/cdrom/preseed/mint.seed boot=casper xforcevesa iso-scan/filename=/boot/iso/linuxmint-13-cinnamon-dvd-32bit.iso ramdisk_size=1048576 root=/dev/ram rw noapic noapci nosplash irqpoll --
    initrd    (iso)/casper/initrd.lz
}
menuentry "Check the integrity of the medium" {
    loopback iso /boot/iso/linuxmint-13-cinnamon-dvd-32bit.iso
    linux    (iso)/casper/vmlinuz  boot=casper integrity-check iso-scan/filename=/boot/iso/linuxmint-13-cinnamon-dvd-32bit.iso quiet splash --
    initrd    (iso)/casper/initrd.lz
}
menuentry "Start Linux Mint 64-bit" {
    set gfxpayload=keep
    loopback iso /boot/iso/linuxmint-13-cinnamon-dvd-64bit.iso
    linux    (iso)/casper/vmlinuz  file=/cdrom/preseed/mint.seed boot=casper iso-scan/filename=/boot/iso/linuxmint-13-cinnamon-dvd-64bit.iso quiet splash --
    initrd    (iso)/casper/initrd.lz
}
menuentry "Start Linux Mint 64-bit (compatibility mode)" {
    loopback iso /boot/iso/linuxmint-13-cinnamon-dvd-64bit.iso
    linux    (iso)/casper/vmlinuz  file=/cdrom/preseed/mint.seed boot=casper xforcevesa iso-scan/filename=/boot/iso/linuxmint-13-cinnamon-dvd-64bit.iso ramdisk_size=1048576 root=/dev/ram rw noapic noapci nosplash irqpoll --
    initrd    (iso)/casper/initrd.lz
}
menuentry "Check the integrity of the medium" {
    loopback iso /boot/iso/linuxmint-13-cinnamon-dvd-64bit.iso
    linux    (iso)/casper/vmlinuz  boot=casper integrity-check iso-scan/filename=/boot/iso/linuxmint-13-cinnamon-dvd-64bit.iso quiet splash --
    initrd    (iso)/casper/initrd.lz
}
##############################################################################

Step 3
------------
Install xorriso using the following command:

$ sudo apt-get install xorriso

To get the latest version, you may also download the source from http://www.gnu.org/software/xorriso and compile.

Step 4
------------
Now step one folder before 'mint-multiboot' and you can generate the multiboot iso using the following command:

$ grub-mkrescue -o Mint-multiboot-dvd.iso mint-multiboot

Step 5
------------
Burn the iso to a DVD or use imagewriter to create a live usb. You should now be able to select 32-bit or 64-bit at boot.