gistfile1.txt

in kv:

<CustomWidget1>:
        id: custom_widget_1
        size_hint_y: None
        height: 0

<CustomWidget2>:
        size_hint_y: None
        Button:
                on_press: root.other_widget.call()


in main.py:

class CustomWidget1(BoxLayout):
    def call(self):
        print('test')

class CustomWidget2(BoxLayout):
    other_widget = ObjectProperty(None)

class ExampleApp(App):
    def build(self):
        wid1 = CustomWidget1()
        wid2 = CustomWidget2()
        wid2.other_widget = wid1

        s = ScreenManager()
        s1 = Screen()
        s1.add_widget(wid1)
        s2 = Screen()
        s2.add_widget(wid2)

        return s