This paper explores a new paradigm for the coexistence among heterogeneous multi-hop networks in unplanned deployment settings, called cooperative interference mitigation (CIM). CIM exploits recent advancements in physical layer technologies such as technology-independent multiple output (TIMO), making it possible for disparate networks to cooperatively mitigate the interference to each other to enhance everyone's performance, even if they possess different wireless technologies. This paper offers a thorough study of the CIM paradigm for unplanned multi-hop networks. We first show the feasibility of CIM among heterogeneous multi-hop networks by exploiting only channel ratio information, and then establish a tractable model to accurately characterize the CIM behaviors of both networks. We also develop a bi-criteria optimization formulation to maximize both networks' throughput, and propose a new methodology to compute the Pareto-optimal throughput curve as performance bound. Simulation results show that CIM provides significant performance gains to both networks compared with the traditional interference-avoidance paradigm.