main.py

# firstbox.py
from kivy.lang import Builder
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import StringProperty

kv = """
<FirstBox>:
    Button:
        text: 'First box: send message to second box'
        on_release:  root.dispatch('on_release')
    Label:
        text: root.text
"""

Builder.load_string(kv)


class FirstBox(BoxLayout):
    __events__ = ['on_release'] 
    text = StringProperty('First Box initial value')

    def on_release(self):
        pass
      
      
# secondbox.py
from kivy.lang import Builder
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import StringProperty

kv = """
<SecondBox>:
    Button:
        text: 'Second box: send message to second box'
        on_release: root.dispatch('on_release')
    Label:
        text: root.text
"""

Builder.load_string(kv)


class SecondBox(BoxLayout):
    __events__ = ['on_release']
    text = StringProperty('second Box initial value')

    def on_release(self):
        pass
      
      
      
# main.py
from kivy.app import App
from kivy.lang import Builder
import firstbox
import secondbox

kv = """
BoxLayout:  # root widget
    orientation: 'vertical'
    FirstBox:
        id: fb
        on_release: sb.text = 'sent from first box!'
    SecondBox:
        id: sb
        on_release: fb.text = 'sent from second box'
"""

class ClasstoClassApp(App):
    def build(self):
        return Builder.load_string(kv)


ClasstoClassApp().run()