You could use a ContentControlOnExit macro in the 'ThisDocument' code module of the document or its template, coded like:
Code:
Private Sub Document_ContentControlOnExit(ByVal CCtrl As ContentControl, Cancel As Boolean)
Dim i As Long
With CCtrl
If .Title = "Master" Then
If .Range.Text = "N/A" Then
For i = 1 To 2
With ActiveDocument.SelectContentControlsByTitle("Slave")(i)
.Type = wdContentControlText
.Range.Text = "N/A"
.LockContents = True
End With
Next
Else
For i = 1 To 2
With ActiveDocument.SelectContentControlsByTitle("Slave")(i)
If .Type = wdContentControlText Then
.LockContents = False
.Range.Text = ""
.Type = wdContentControlDropdownList
End If
End With
Next
End If
End If
End With
End Sub
In the above code, the controlling content control is titled 'Master' and the two dependent content controls are titled 'Slave'.