This tutorial explains how to create a multiboot USB pen drive, enabling users to boot from different operating systems on one device.
Next, you can proceed with the following steps: mount the USB pen where the first grml ISO is installed using grml2usb, mount the second ISO, set up squashfs, and copy the kernel and initrd. Once you're done with these steps, adjust the syslinux.cfg file and add an additional entry for the second ISO.
After making the necessary adjustments, you can finally invoke syslinux to complete the setup. Once done, you'll have a USB pen that allows you to choose "grml64" on the bootprompt to run the 64bit version of grml.
This is a pretty straightforward and simple process, and the tutorial does a great job of explaining every step in-detail. If you need to run both 32 and 64bit versions of grml on a single USB pen, this tutorial is definitely worth checking out.