The selected value of a list control is the range.text. You can identify a control by its tag which is by default the same as its title, though either can be changed as required. Frankly it is difficult to see why you would put the list value in a text CC when it is already displayed in the list CC, but if you wish to do so then use the ControlOnExit event to make the update e.g. in your document.
Code:
Option Explicit
Private Sub Document_ContentControlOnExit(ByVal cc1 As ContentControl, Cancel As Boolean)
If cc1.ShowingPlaceholderText = False Then
ActiveDocument.SelectContentControlsByTitle("a").Item(1).Range.Text = cc1.Range.Text
End If
End Sub