A model is presented on pairs of exchange-coupled magnetic layers that have interfaces with surface anisotropy. The interfaces are coupled to the bulk by twisted magnetization configurations. Experimental magnetization curves of Co/Ru sandwiches are reproduced with precision. Most of the physical parameters used are in accordance with the literature. A difference in the anisotropy constants between both magnetic layers was found, which, in the present samples, can be explained by lattice misfit and interface roughness. The exchange coupling forces the magnetization of both layers to be along the same axis in the low field range.