I'm currently developing on Google Tango using Unity. Currently when I add Tango Delta camera and Tango AR camera, only one camera can be used,when I tried with motion tracking and depth perception, the real environment cannot be seen. Is it possible to achieve depth perception using the AR camera?
Yes, Turning Depth Perception On/Off isn't controlled by the Camera you use, but by a setting on the "Tango Manager".
The camera you select isn't enabling or disabling Tango core features, it's just displaying stuff in different ways. You could use the Delta camera with Motion tracking enabled but Depth disabled, for example.
Open your Tango Manager in the Inspector and look for "Enable Depth".
Several of the Unity Examples use Depth alongside the AR Camera.
You can also check out the Depth Perception Tutorial