By following this tutorial is possible to get a multiboot USB pen, with grml 1.1, grml64 0.2 and grml-medium. This might be useful if you need a 32bit together with a 64bit version on just one medium like a USB pen.

First of all take the 32bit version and install it to your USB pen using grml2usb. Then proceed as follows:

# Mount the USB pen (in this example /dev/sdb1) where you installed
# the first grml ISO using grml2usb:
GRML2USB=/mnt/usb-sdb1
mount ${GRML2USB}

# Mount the second ISO as well:
TARGET=/mnt/test
mount -o loop /grml/grml-isos/grml64_0.2.iso ${TARGET}

# Set up squashfs:
cp ${TARGET}/live/grml64.squashfs ${GRML2USB}/live/grml64.squashfs
echo grml64.squashfs >> ${GRML2USB}/live/grml64.module

# Copy kernel and initrd:
cp ${TARGET}/boot/grml64/initrd.gz ${GRML2USB}/grml64.gz
cp ${TARGET}/boot/grml64/linux26 ${GRML2USB}/grml64

Now adjust syslinux.cfg and add an additional entry for the second ISO. Usage example:

# cat syslinux.cfg
[...]
# Configuration for 32bit grml (being present already):
LABEL grml
MENU LABEL grml - Standard Bootup
MENU DEFAULT
KERNEL linux26
APPEND apm=power-off lang=us vga=791 quiet initrd=initrd.gz boot=live nomce

# Add an additional configuration entry for 64bit grml:
LABEL grml64
MENU LABEL grml - Standard Bootup
MENU DEFAULT
KERNEL grml64
APPEND apm=power-off lang=us vga=791 quiet initrd=grml64.gz boot=live module=grml64 nomce
[...]

Finally invoke syslinux (don’t forget to adjust the device name according to your needs):

syslinux /dev/sdb1

That’s it! Now you should have a USB pen which lets you choose “grml64” on the bootprompt to boot the 64bit version of grml.