Bassar B’chalav is not really Hetter B’hetter. The idea of Hetter B’hetter not being Batul is that there is nothing to be Mevatel. Bittul is more a Din than a Metzius. Bassar in Chalav is an Issur.
Bassar Bechalav, on the other hand, should really be Assur Bemashehu. Just as in Klaim we say that the Iruv is Assur and therefore there is no Bittul, Bassar Bechalav would be the same, if not for the fact that we need Nesinas Taam.