Microbial communities are commonly observed in nature. It has been observed that such communities seem to be more efficient in use of resources as well as more robust, than monocultures. We study a simple synthetic microbial community, where two metabolic pathways of E.coli are split into two different strains of E.coli. Thorough analysis of a model shows that the total biomass of the consortium must be smaller than that of monoculture under very general conditions on the shape of the growth curves. This contradicts the experimental observations. To explain this observation, we conclude that some adaptation must have occurred in the members of the consortium.