Because RF nonlinear systems exhibit dynamic behaviors that are not only dependent on the central frequency (direct memory) but also on the envelope dynamics (cross memory), characterization metrics have been an object of research for many years. Unfortunately, as the response of these systems depends on the excitation, a general metric requires complex stimulus and cumbersome measurements. This work uses the fact that a standard two-tone excitation is already sufficiently complex to excite the most important nonlinearity and memory in nonlinear RF systems, as a step towards the development of such a metric. As an example, simple two-tone excitation measurements are used to evaluate both the direct and cross memory of an RF PA in a large bandwidth around a carrier frequency, and are then used to build a quasi-orthogonal GMP-like model.