A ubiquitous and high-performance sensing service is required in the next generation of wireless communication systems. The legacy Sub-6GHz bands can provide a larger sensing range, but the available bandwidth limits the resolution. In contrast, the millimeter-wave (mmWave) frequencies enhance the sensing performance with wider bandwidth. However, high-directional beamforming poses new challenges in passive target surveillance. The conventional beam sweeping in mm Wave is costly due to the significant overhead, which conflicts with the goal of communication on efficient data transmission. To reduce the overhead caused by target surveillance, we propose a dual-band sensing approach. By combining the advantages of both Sub-6GHz and mmWave frequencies, our method enables fine target detection in a wide area. Compared to beam sweeping, the pro-posed method significantly reduces the communication overhead and time consumption, while also lightening the computational load of radar signal processing.