Inherited bacteria Wolbachia, and more recently Cardinium, have received a great deal of attention for their ability to manipulate the reproduction and fitness of their host species. Wolbachia and Cardinium have been found to co-infect the same host species. In this study, both Wolbachia and Cardinium were found to manipulate host reproduction through cytoplasmic incompatibility and to affect the male-biased sex-ratio in the doubly infected spider mite Tetranychus truncates Ehara. We also investigated effects of double infection with Wolbachia and Cardinium on host fecundity and longevity. Results indicated that Wolbachia and Cardinium increased the fecundity of doubly infected females, although no infection effect on host longevity was observed. Our most important finding was that the mating of uninfected mites facilitated the proliferation of Wolbachia and Cardinium in double-infected mites. We discuss the results observed with respect to the spread of bacterial infection in natural populations and the evolution of the endosymbiont—T. truncates symbiosis.