Microbial communities primarily consist of multiple species that affect one another's fitness both directly and indirectly. This study showed that the cocultivation of Paenibacillus amylolyticus and Xanthomonas retroflexus exhibited facultative mutualistic interactions in a static environment, during the course of which a new adapted phenotypic variant of X. retroflexus appeared. Although the emergence of this variant was not directly linked to the presence of P. amylolyticus, its establishment in the coculture enhanced the productivity of both species due to mutations that stimulated biofilm formation. The mutations were detected in genes encoding a diguanylate cyclase predicted to synthesise cyclic-di-GMP. Examinations of the biofilm formed in cocultures of P. amylolyticus and the new variant of X. retroflexus revealed a distinct spatial organisation: P. amylolyticus only resided in biofilms in association with X. retroflexus and occupied the outer layers. The X. retroflexus variant therefore facilitated increased P. amylolyticus growth as it produced more biofilm biomass. The increase in X. retroflexus biomass was thus not at the expense of P. amylolyticus, demonstrating that interspecies interactions can shape diversification in a mutualistic coculture and reinforce these interactions, ultimately resulting in enhanced communal performance.