best way to get the list of unique keys from 2 alists

[Kevin Rodgers <ihs_4664@yahoo.com>]

I've got 2 alists of (SYMBOL . "STRING") pairs, and I need to get the
list of unique symbol names to pass to completing-read as its TABLE
argument: (("SYMBOL-NAME") ...)

I know about remove-duplicates and union, but I'd like to avoid using
cl*.el functions.

Since completing-read seems to ignore nil entries in TABLE, this is what
I've got now:

(nconc (mapcar (lambda (assoc)
                  (list (symbol-name (car assoc))))
                alist-1)
        (mapcar (lambda (assoc)
                  (or (assq (car assoc) alist-1)
                      (list (symbol-name (car assoc)))))
                alist-2))

Is there a cleaner way?

Thanks,
-- 
Kevin