I am afraid your code is dangerous - you are adding an events every time the textbox gets the focus and the user can still select the text box. Try this
Code:
Sub Globals
'Declare the global variables here.
End Sub
Sub App_Start
Form1.Show
End Sub
Sub cmdShow_Click
txtReadOnly.Text=txtReadWrite.Text
End Sub
Sub txtReadonly_GotFocus
txtReadWrite.Focus
End Sub