The concept of a group of checkbox content controls working in the same way as the old group of radio buttons (only allowing one active selection) is tricky to code. There may be a more elegant way to do this but my attempt is more flexible in supporting more than two options.
Code:
Private Sub Document_ContentControlOnExit(ByVal aCCactive As ContentControl, Cancel As Boolean)
Dim aCC As ContentControl
Select Case aCCactive.Title
Case "MyCheck", "Q1"
If aCCactive.Checked Then
For Each aCC In ActiveDocument.SelectContentControlsByTitle(aCCactive.Title)
aCC.Checked = False
Next aCC
aCCactive.Checked = True
End If
End Select
End Sub