I had another look at the source code, and found this:

           (concat label " a <symbol> or (<symbol or list of symbols> . <symbol or function>)"
                   " or list of these")

It's unfortunate that this is part of an error message, as I've never seen the error, but I think this explains the syntax, and indeed says that you can't add multiple functions to a single hook with one :hook line; in other words, it would be necessary to write something like:

(use-package foo-mode

   :hook

   ((foo-mode . function-a)

   (foo-mode . function-b)))