Doping control at the nanoscale can be used to modify optical and electronic properties thus inducing interesting effects that cannot be observed in pure systems. For instance, it has been shown that luminescence energies in silicon nanocrystals can be tuned by properly controlling the impurities, for example by boron (B) and phosphorus (P) codoping. Starting from hydrogen-terminated silicon nanoclusters, we have previously calculated from first-principles that codoping results are always energetically favored with respect to single B- or P-doping and that the two impurities tend to occupy nearest neighbor sites near the surface. The codoped Si nanoclusters present band-edge states localized on the impurities which are responsible for the red-shift of the absorption thresholds with respect to that of pure undoped Si nanoclusters. Here we investigate how the properties of the codoped nanoclusters are influenced by adding one or two more impurities. Moreover we study also the effect of B- and P-codoping on the electronic and optical properties of Si nanowires, thus investigating the role of dimensionality, 0- versus 1-dimensionality, of the systems.