The classical wave-particle Hamiltonian is considered in its generalized version, where two modes are assumed to interact with the co-evolving charged particles. The equilibrium statistical mechanics solution of the model is worked out analytically, both in the canonical and the microcanonical ensembles. The competition between the two modes is shown to yield ensemble inequivalence, at variance with the standard scenario where just one wave is allowed to develop. As a consequence, both temperature jumps and negative specific heat can show up in the microcanonical ensemble. The relevance of these findings for both plasma physics and Free Electron Laser applications is discussed. [6]. For these systems energy is non additive implying that entropy could be non concave in some range of values of macroscopic extensive parameters [7]. This is at the origin of ensemble inequivalence, which in turn instigates peculiar thermodynamic properties, like negative specific heat and temperature jumps in the microcanonical ensemble. Ensemble inequivalence has been reported to occur in the past for gravitational systems [8], spin models [9] and twodimensional flows [10]. These are extremely interesting models per se, as well as for their theoretical implications, but in general they do not allow for a straightforward experimental verification of the predictions drawn.A paradigmatic example of long range interacting system is represented by the so called wave-particle Hamiltonian, which describes the self-consistent interaction of N charged particles with a co-evolving wave [11]. This is a rather general descriptive scenario, often invoked in different fields of investigations where the mutual coupling between particles and waves proves central, as e.g. in plasma physics [12] and the Free Electron Laser (FEL) [13]. The equilibrium statistical mechanics solution of the celebrated wave-particle Hamiltonian has been so far solely carried out for the simple setting where just one isolated wave is allowed to exist [14,15]. Working under this limiting, and in many respects, unrealistic assumption, it can be shown that the canonical and microcanonical solutions coincide [16]. In real experiments, however, several modes are simultaneously present and interact with the bunch of co-evolving particles. Motivated by this observation and to eventually bridge the gap between theory and experiments, we here consider the generalized equilibrium solution of the reference wave-particle Hamiltonian when a second harmonic is allowed to self-consistently develop. As we shall analytically demonstrate, the insertion of an additional wave causes the ensemble inequivalence to rise, within a specific range of the coupling constants which yield a first order phase transition in the canonical ensemble. This conclusion is reached for a model of marked experimental value, paving the way to direct verifications and, possibly, exploitations of the general concept of ensemble inequivalence.The starting point of our discussion is the universal wave particle Hamil...