  We present the results of directional point-contact spectroscopy in
state-of-the-art Mg_{1-x}Al_xB_2 and Mg(B_{1-y}C_{y})_2 single crystals
produced at ETH, Zurich. Fitting the conductance curves of our point contacts,
that always feature Andreev reflection structures, we obtained the doping
dependence of the gap amplitudes. The results are discussed in comparison with
other experimental findings and relevant theoretical predictions. We conclude
that the physics of Al-substituted crystals at x > 0.09 might be governed by
phase segregation, while C-substituted crystals unexpectedly show a
doping-induced transition to single-gap superconductivity at y=0.132.
