[en] The critical current density in industrial Nb₃Sn and MgB₂ wires is currently optimized by introducing various kinds of additives, either Ta and/or Ti for Nb₃Sn wires or SiC or C for MgB₂ wires. In the following, several problems linked to the presence of additives in the two classes of compounds are discussed. A reinvestigation of the site occupancy of Ta and Ti additives in Nb₃Sn wires shows that the Ta atoms occupy the 6c chain sites, while the Ti atoms are located on the cubic 2a sites. It follows that in perfectly ordered A15 compounds A_1-βB_β, the relation ρ_o versus β exhibits a 'universal' behavior: the effect of the chemical nature of the constituents on ρ_o is negligible. The slopes of ρ₀ versus the Ti, Ga and Ni contents in the A15 layer coincide and are much steeper than for the Ta additive, corresponding to the three times higher number of 6c sites with respect to 2a A15 lattice sites. The presence of two grain morphologies, e.g. equiaxial and columnar, is observed in Nb₃Sn wires produced by the bronze route only. The nonlinearity of the Kramer plot in multifilamentary Nb₃Sn bronze route wires is explained by the presence of these two different grain types, which have distinctly different Sn contents and sizes. For these wires, the total pinning force can be represented as the superposition of two contributions with different scaling fields. Simultaneous addition of different additives on 'in situ' Fe/MgB₂ wires is presented as an attempt to combine different possible mechanisms influencing J_c. The substitution of boron by carbon is known to enhance the value of ρ_o and thus of the critical field. In addition, the pinning behavior is expected to be improved by grain boundary effects or nanosize precipitations, caused by the presence of appropriate additives during the MgB₂ phase formation. Since the two mechanisms are independent, their effect on J_c is expected to be cumulative. In the present paper, the results on the additive combination B₄C+LaB₆ in monofilamentary Fe sheathed MgB₂ wires are reported. The data are compared with the additives B₄C+SiC and show that simultaneous additives could be promising in view of applications at 20 K