Are you booting the cd everytime and wanting different save files on the pendrive for each user?
If so. Just name the save files to the new users name and save as ext2. I'd format the USB as ext2 since
ext3 journaling file systems write too much to those cheap flash drives and prematurely wears them out.
If installing to pendrive, (If me I'd format it as ext2). Use
to run a virgin session in ram and when it asks to save. Use the user name that ran that live session.
On any next boot whether running live cd or installed pendrive. A list of save files will come up that you made for each user.
Just pick the number 1,2,3,4 whatever is next to the name and it will boot that save file only.
My SD card install
My save file is ext2 and sits on a ntfs file system since I run Puppy on my motorcycle tuner laptop with Windows 7 on it.
I did not feel like dual booting and I needed to make use of a 128MB Flash drive.