The vision of creating a "virtual supercomputing" environment to solve large-scale scientific problems has largely been facilitated by the development and deployment of Grid middleware. However, with the deployment of multiple disconnected Grid environments, we are now faced with the problem of interoperable access to resources from multiple environments to meet the requirements of scientific applications. Within the U. S. cyberinfrastructure environments, two key elements: both the National Science Foundation TeraGrid and the Open Science Grid (OSG) provide varied but important capabilities and resources needed by diverse computational communities. Hence, it is critical to understand how these communities can benefit from bridging these different environments and utilize them when needed. In this paper we present a novel approach to interoperable access to both OSG and TeraGrid to users through the CyberGIS Gateway - an online geographic information system. In particular, five key interoperability themes are addressed: authentication and authorization, information services, data management, and computation management and auditing. We take a scientific application use-case (viewshed analysis) on the CyberGIS Gateway to demonstrate how to exploit resources on both OSG and TeraGrid.