In this paper, the transmission capacity of two existing wireless networks (the primary (PR) network and the secondary (SR) network) operating in the same geographical region and sharing the same spectrum is studied. We define transmission capacity as a product of the following signal propagation parameters: the density of transmission, the transmission rate and transmission outage probability. The PR network has a higher priority to access the spectrum regardless of the SR network, while the SR network limits its interference on PR network by carefully controlling the density of the transmitters. We quantify the transmission capacity for both networks and study their tradeoffs using computer simulation. Simulation results show that the proposed model has minimized mutual interference by significantly increasing the network spectrum efficiency per unit area and the transmission capacity.