Today's mobile devices support many wireless technologies to achieve ubiquitous connectivity. Economic and energy constraints, however, are driving the industry to implement multiple technologies into a single radio. This system-on-a-chip architecture leads to competition among networks when devices toggle across different technologies to communicate with multiple networks. In this paper, we study the impact of such network competition using a representative scenario where devices split their time between WiMAX and WiFi connections. We show that competition with WiMAX significantly lowers WiFi's throughput, but this performance degradation is largely unnecessary, and can be attributed to the fact that WiMAX's transmission scheduling does not consider competing networks. We propose PACT, a new coexistence-aware WiMAX scheduling policy that cooperates with WiFi links hosted by its users without compromising its own transmission requirements. We derive PACT's design using an analytical model of network competition, and apply it to design practical WiMAX scheduling algorithms for various traffic classes. We evaluate PACT using OPNET's realistic models for WiFi and WiMAX. Using real network topologies, our experiment results show that PACT significantly improves WiFi performance by up to 17 fold without affecting the WiMAX user experience.