I have successfully created a bootable Live USB stick on an 8Gb drive with either the 32 and 64 bit version of 11.10 but not bother at the same time. I would like to put them both on the same stick along with a third distribution (DSL) so I only have to carry one drive. How do I modify the GRUB files to make this happen?
I can see why one might want both, especially if you take it around to multiple machines and it's for testing purposes.
For my multiboot sticks I use a program called Multisystem. It takes care of the grub configuration for you and makes it easy to remove and add various distros.
There is a document describing how to do this on the Ubuntu wiki:
Basically, you configure grub to show a menu of .iso images on the USB device, "loopback mount" the right one, then boot from that.