I would use
Create Library With the Borrow Structure option
to make an empty Library on each USB stick
So I don't miss any
I would Select a bunch (A-M, M-Z) of AUTHORS in the tag browser
(green plus)
Select ALL results (ctrl-A)
Right click: copy to Library: <USB stick #>
repeat for the second group
Remember to copy your User configuration folder
to bad,
Export All Calibre data can't offer a Spanned Volumes Zips