This work proposes a novel crossover cavity with high isolation and power handling capability. It consists of a one-pole Substrate Integrated Waveguide (SIW) circular resonator that excites two decoupled evanescent modes at the same resonant frequency. The proposed structure deploys two substrates acting as one evanescent-mode circular cavity. The electric fields of the evanescent modes are confined in the secondary substrate. Meanwhile, most of their magnetic fields exist in the main substrate. Each semicircular post is loaded by a capacitive load ($C_{1}$) in the secondary substrate, defining the resonant frequencies of the excited modes. Moreover, the isolation between channels is controlled through capacitive slots in the top metal layer of the main substrate. Consequently, the resonant frequency of the proposed crossover cavity and the isolation between its channels are independently tuned. The proposed SIW crossover cavity is implemented at 8.7 GHz with a size of (0.65$\lambda_{g}\times $0.65$\lambda_{g}$). A high-power handling capability of 44.7 dBm is measured. Also, 1.7 dB and 64 dB levels of insertion loss and isolation are measured, respectively. Furthermore, a 50-dB isolation bandwidth of 739 MHz is achieved. The scalability of the one-pole crossover cavity to a second-order transfer function has been demonstrated in a simulated two-channel crossover configuration.