  We evaluate exclusive semileptonic decays of ground-state spin-1/2 and
spin-3/2 doubly heavy $cb$ baryons driven by a $c\to s,d$ transition at the
quark level. We check our results for the form factors against heavy quark spin
symmetry constraints obtained in the limit of very large heavy quark masses and
near zero recoil. Based on those constraints we make model independent, though
approximate, predictions for ratios of decay widths.
