Old topic, I know, just wanted to add my experience here since my solution differs, and because it took me so long to figure out.
I wanted Ubuntu (I used 8.10, the current one as of this post) and UBCD to boot from the same USB stick. I installed UBCD first (via this guide
) and ensured it was bootable.
Next, following the tips in this thread, I copied all the files and folders from the contents of the Ubuntu ISO except for the /isolinux folder. I copied the contents of the Ubuntu /isolinux folder to my UBCD /syslinux folder, renaming the copied isolinux.cfg to /syslinux/ubuntu.cfg.
This was where I hit problems. Merely appending
MENU LABEL ^Ubuntu
...as per Icecube's suggestion in this thread in my custom.cfg file would not boot! In fact, it didn't do anything; it simply sat there with the line selected. After toying around, my FINAL custom.cfg file looks like this:
MENU INCLUDE /menus/defaults.cfg
MENU LABEL ..
MENU LABEL Ubuntu 8.10 LiveCD
But this by itself is not quite enough! Syslinux would complain at me that the menu file I wanted to use had no LABEL arguments. I checked, and sure enough, it didn't, but it included a file (menu.cfg) that included a file (text.cfg) that had the relevant LABEL arguments to boot. So my solution was to replace the line "include text.cfg" (sans quotes) in menu.cfg with the entire contents of text.cfg, and then to replace the line "include menu.cfg" in ubuntu.cfg (formerly isolinux.cfg, remember) with the entire contents of menu.cfg, including the whole of text.cfg.
This may be a kludgy solution, but it works for me with no visible side effects (except that the Ubuntu boot menu uses the UBCD style... so what?). Ubuntu works just fine, and so does UBCD from all I've seen. Hope that helps someone!