arcpy - Dynamically adding items into one python addin Combobox from another?

I have two combo boxes. I am trying to add items dynamically into the second combobox from the first combobox's onSelChange(self, selection) event. due to some reason, this is not working.

Here is my sample code:

import arcpy
import pythonaddins


class ComboBoxClass1(object):
    """Implementation for TestCombo_addin.combobox (ComboBox)"""
    def __init__(self):
        self.items = ["cb1item1", "cb1item2"]
        self.editable = True
        self.enabled = True
        self.dropdownWidth = 'WWWWWW'
        self.width = 'WWWWWW'
        self.cb2=ComboBoxClass2()


    def onSelChange(self, selection):
        pythonaddins.MessageBox(selection,"Message",0)
        self.cb2.items.append(selection)
        self.cb2.refresh()
    def onEditChange(self, text):
        pass
    def onFocus(self, focused):
        pass
    def onEnter(self):
        pass

    def refresh(self):
        pass

class ComboBoxClass2(object):
    """Implementation for TestCombo_addin.combobox_1 (ComboBox)"""
    def __init__(self):
        self.items = ["cb2item1", "cb2item2"]
        self.editable = True
        self.enabled = True
        self.dropdownWidth = 'WWWWWW'

        self.width = 'WWWWWW'

    def onSelChange(self, selection):
        pass
    def onEditChange(self, text):
        pass
    def onFocus(self, focused):
        pass
    def onEnter(self):
        pass

    def refresh(self):
        pass

Answer

I got a solution (Thanks to Freddie Gibson from ESRI.) I am curious to know if there are other solutions.

import arcpy
import pythonaddins

class ComboBoxClass1(object):
    """Implementation for TestCombo_addin.combobox (ComboBox)"""
    def __init__(self):

        self.items = ["cb1item1", "cb1item2"]
        self.editable = True
        self.enabled = True
        self.dropdownWidth = 'WWWWWW'
        self.width = 'WWWWWW'
        ComboBoxClass1._hook=self
    def onSelChange(self, selection):
        pass

class ComboBoxClass2(object):

    """Implementation for TestCombo_addin.combobox_1 (ComboBox)"""
    def __init__(self):
        self.items = ["cb2item1", "cb2item2"]
        self.editable = True
        self.enabled = True
        self.dropdownWidth = 'WWWWWW'
        self.width = 'WWWWWW'
    def onSelChange(self, selection):
        ComboBoxClass1._hook.items.append(selection)