GroovyFx binding to two properties

[Tx. T]

how would I go about binding the textProperty of two textfields to a button's 
disable (on/off)?  Here is an example of binding to 1 textfield.  I like to 
know how to do similar but with 2 or more textfields.Basically, if two 
textfields are empty, the submit button is disable, else.#!/usr/bin/env groovy

import static groovyx.javafx.GroovyFX.start

start {
    stage title: "Test App", visible: true, {
        scene {
            hbox {
                tf = textField(id: "tf")
                button('Submit', id: "smb", disable: 
bind(tf.textProperty()).using { it.isEmpty() })
            }
        }
    }
}