In the conventional Rashba model for an isotropic two-dimensional electron gas (2DEG), the electrons are spin-orbit split by a structural inversion asymmetry (SIA) perpendicular to the confinement plane. An additional SIA within the confinement plane leads to another contribution to the spin-orbit interaction which is investigated by means of a nearly-free electron model. The interplay of both contributions manifests itself as an enhanced splitting in the anisotropic 2DEG, as compared to the isotropic case. Further, the spin polarization of the electronic states is rotated out of the confinement plane. Both findings corroborate recent experimental and theoretical results for the ordered surface alloys Bi/Ag(111) and Pb/Ag(111).