Researchers often couple different, formerly independent simulators to form a heterogeneous simulation environment, but often choose tight coupling or an overly generic approach, each of which has its drawbacks. In this paper, we propose a generic middleware-based interface to couple independent simulators for federated network simulation, which strikes a new balance. Our co-simulation approach performs event aggregation over a small time step to reduce the cost of frequent inter-federate interactions. We also perform a runtime evaluation to estimate the overhead introduced by our co-simulation approach.