This paper presents a novel overlay architecture to allow the design and development of distributed applications based on multiple interconnected overlay networks. Message routing between overlays is achieved via co-located nodes, i.e. nodes that are part of multiple overlay networks at the same time. Co-located nodes, playing the role of distributed gateways, allow a message to reach a wider set of peers while overlay maintenance remains localized to individual overlays of smaller size. To increase robustness, gateway nodes route messages in an unstructured fashion, and can discover each other by analyzing the overlay traffic. The approach is able to work in both "collaborative" scenarios, where overlay protocol messages can be modified to include additional inter-routing information, or non-collaborative ones. This allows for the interaction with existing overlay protocols already deployed.