An occasion arose for completion-table-in-turn but disappointed to
discovered this deficiency.

1. Emacs -q -l t.el
2. M-x t-setup
3. type TAB till the *Completions* buffer pops up

I saw:

  Possible completions are:
  abc
  def
  ghi

The other table is ignored.

Completion in the buffer still works for the second table. But
minibuffer-completion-help provides little help in this case.

Leo