[racket] racket/gui : How to properly code a two window system ?

Hello,

I have two files :
	* List-item.rkt : Display through a list-box% some items
	* add item.rkt : Display a form to add an item


add-item window has different text-field% and a button% (label "ok")
that create a new item stored in a variable foo and (send add-item show #f)

In the main window (list-item), I have a button% (label "Add"), and in
the (callback) i would like to say something like :

(send add-item show #t)
; while add-item frame show equal #t, pause.
(send list-item add foo) ; just for example to simplify.

I dont think it's a good algo to perform that. My question is about :
how to send data created by add-item.rkt to List-item.rkt in the
callback from the "add" button.

Thanks in advance.