The two title compounds were synthesized and investigated with the inelastic-neutron-scattering (INS) technique. They contain mixed YbMBr₉^3- (M=Cr³⁺, Ho³⁺) dimers as discrete units, and the magnetic excitations of mixed Yb³⁺-Cr³⁺ and Yb³⁺-Ho³⁺ dimers could thus be observed. The Yb³⁺-Cr³⁺ dimer has three INS transitions, for which anisotropic exchange, as well as zero-field splitting of Cr³⁺, has to be included in the exchange Hamiltonian. For the Yb³⁺-Ho³⁺ dimer the effect of the exchange interaction manifests itself as a broadening and a splitting of the crystal-electric-field levels of the isolated Ho³⁺ ion. Taking into account the full (2J + 1) ground-state multiplet of Ho³⁺, as well as anisotropic exchange, gives a satisfactory description of this dimer.