The problem is that you declare the variables 'cmb1 and'cmb2
inside the scope (context) of a method, the Button Click EventHandler; outside of that method, therefore, the instances of the ComboBoxes you created do not exist (well, they exist, in a way, until they are garbage collected automatically).
Just move the declaration of your ComboBoxes out of the method, and place them in the scope (context) of the Form:
ComboBox cmb1 = new ComboBox();
ComboBox cmb2 = new ComboBox();
private void button1_Click(object sender, EventArgs e)
{
}