Simultaneous overlay networks compete for network resources and disrupt each other. If they cooperate with each other, the collective performance can be improved and they can coexist comfortably. Taking inspiration from biology, in this paper we present a model of symbiotic overlay networks. Coexisting overlay networks dynamically evolve, interact with each other, and change their internal structures. Overlay networks in a symbiotic condition eventually establish the strong relationship and finally merge into one. We also evaluate the effect of interconnection of two overlay networks from the viewpoint of the robustness and the rate of message dissemination.