You will need to expose your RadButton as a property on UserControl1. Then subscribe to the click event on that button in your main form that contains UserControl1. Once clicked you will need to create UserControl2 then add it to the controls collection on the RadPanel.
public partial class UserControl1 : UserControl
{
public UserControl1()
{
InitializeComponent();
}
public RadButton Button
{
get { return radButton1; }
}
}
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
userControl1.Button.Click += new EventHandler(Button_Click);