User communities have gathered around various Grid systems forming separate islands that represent borders they cannot cross. As these communities are growing and demanding more and more computational power, uniting these islands draws more attention in Grid research and development. This problem is called the Grid Interoperability problem. This issue may be tackled at different levels in the Grid middleware, this paper targets the level of Grid resource management also called as Grid brokering. This paper introduces a meta-brokering approach that means a higher level resource management by enabling automatic and simultaneous utilization of various Grid brokers managing resources of different Grid systems. Desktop Grids are using not only different interfaces, but also different technologies. Here we also introduce a novel solution to access Desktop Grids through specific brokers that can be managed by our Meta-Broker. In this way with our multi-level brokering solution we can unify several Service and Desktop Grids without modifying their implementation, policies or interfaces, providing the greatest computational power possible for all Grid user communities.