I think the answer to your question is "no" you can't query both .Tag and .Title simultaneously to find the content control of interest.
For what is seems to me you are trying to do though, something like this might do.
Code:
Private Sub Document_ContentControlOnExit(ByVal oCC As ContentControl, Cancel As Boolean)
Dim collCCs As ContentControls
Dim oCCTarget As ContentControl
Select Case oCC.Tag
Case "a"
Set collCCs = ActiveDocument.SelectContentControlsByTag("b")
For Each oCCTarget In collCCs
If oCCTarget.Title = oCC.Title Then
oCCTarget.Range.Select
Exit For
End If
Next
End Select
End Sub