One of the major enabler for future wireless communication technologies is spectrum sharing, allowing to effectively share spectrum portions among different services. Even if much more considered in wireless terrestrial communications, spectrum sharing is increasing importance also in satellite communications where the spectrum scarcity issue is arising; cognitive techniques are a promising tool to cope with this problem. Taking into account the wide coverage and the flexibility that multibeam satellite systems can provide, in this paper we propose an optimization strategy that allows to efficiently assigns spectrum portions and polarizations to the beams. The proposed strategy is based on the definition of a Quality Index and aims at evaluating the beam pattern that maximizes the coverage. Assessments and evaluations of the potential gain in spectrum usage are performed with respect to the considered scenario.