I have used 2,4-D,B (Butyrac) + Select in the same tank mix.... once. I won't do it again.  It set the alfalfa back.  If I want to use them, use them in separate loads.  There is a recommendation which one to apply first - I don't recall whether to apply the Butyrac first, or the Select first.  But it should be several days apart.   I am thinking to apply the Butyrac first, and then the Select a couple of days later.  And don't use any crop oil or surfactant with the Butyrac.