Optical absorption and photoluminescence spectra performed on Ho3+ (4f10) and Er3+ (4f11) doped KGd(WO4)2 single crystals were measured at selected temperatures between 5 and 80 K. In this monoclinic matrix, space group C2/c (No. 15), the lanthanide ions replacing Gd3+ occupy a single crystallographic site with symmetry C2. The interpretation of these spectra and the simulation of the splitting of 2S+1LJ levels were carried out in the frame of the phenomenological single-electron crystal field theory, through the simultaneous diagonalization of the energy matrices of the free-ion (fi) and crystal field (cf) interactions. A total of 20 fi and 14 cf adjustable parameters were considered for the reproduction of experimental energy levels and their associated wavefunctions. The simulated energy level series are in excellent accordance with the experimental data, i.e. rms deviations, [sigma]=14.5 and 15.6 cm-1 for Ho3+ and Er3+, respectively. The comparison with similar results for Pr3+ (4f2) and Nd3+ (4f3) allowed establishment of the variation trends of the cf parameters with the number of 4f electrons.