Here is a thread
I wrote a few days ago telling how to put Grub into floppy, pen drive, CD/DVD or hard disk.
You only get a Grub prompt out of it but there is no PC system I know that cannot be booted up manually
by a Grub prompt. By adding a menu.lst in the /boot/grub directory of the booting media you can boot Oses automatically.
Grub will be able to see USB devices if the USB legacy support is enabled in the Bios.
If you want to put two boot loaders into a USB storage device then have two partitions and put one boot loader in each one.