Depth information is a critical cue for scene understanding which is very important for video surveillance. Traditional depth estimation approaches based on stereo vision have been well studied in past decades. However, little research is conducted with active cameras. In this correspondence, we discuss the depth estimation problem by employing dual-PTZ (pan/tilt/zoom)-camera system. The contributions of this correspondence includes the following three aspects: 1) we analyze the effect on depth's precision with different PTZ settings; 2) we propose a coarse-to-fine framework to deal with depth estimation problem for complex region; 3) we offer a method to generate mosaic of depth maps to combine depth information from different visual angles and resolutions for large scope. These contributions will widen the usage of dual-PTZ-camera systems to a greater extent. Real-data experimental results show that the proposed approach works well.