Upon reboot, with the CD/DVD present inside CD/DVD drive, new GRUB will display list of live distros available in the disc. Note: When you download any Linux iso image from internet it will look like this "ubuntu-9.04-desktop-i386" but this has to be renamed as (inside multicd folder). List of Live Linux distros supported in this script are given below (and the names accepted by this script are on the right side of each distro)." There's a list of distro's on the blog page.UNetbootin allows you to create bootable Live USB drives for Ubuntu, Fedora, and other Linux distributions without burning a CD. You can either let UNetbootin download one of the many distributions supported out-of-the-box for you, or supply your own Linux file if you've already downloaded one or your preferred distribution isn't on the list.SARDU multiboot creator build a multiboot USB drive, a DVD multiboot ISO or a CD multiboot ISO.
If you think you are up to the task, please have a look here.This is the primary question for the thread while I have a few more below for anyone feeling like providing some additional guidance. Secondary questions: Does your flash drive have an actual install of Ubuntu or is it just a Live CD on a flash drive.If it is an install and you have Grub2 installed to the MBR of the flash drive, you could find the /boot/grub/file and put menuentries there and boot the iso files directly from a partition on the flash drive. https://help.ubuntu.com/community/Grub2/ISOBoot I've been making multiboot flash drives for years.Download the executable and run it – no installation is necessary.The executable is digitally signed and the signature should state: If you create a DOS bootable drive and use a non-US keyboard, Rufus will attempt to select a keyboard layout according to the locale of your system.It is really very simple to create multi Live Linux distro in a single disc.