how to use second camera in unity ?

Are you trying to have two cameras showing at once, or swap between two cameras?

am using 2 cameras at a time....... one is front end and another one is top angle

Well, I remember I found a step by step explanation to this, either in the answers or forums or wiki ... Here's the steps:

Create a second cam,in the inspector remove the "Audio Listener" and "Flare Layer" components.

On top of the inspector, there's the layer dropdown, click and select add layer, Write your desired Layer name (user layer 8 must be empty). (this will be the layer of your object you want to show on top of everything).

Select the object (your button) and set its layer to the new layer you created.

Now select the second camera, and in the inspector set the clear Flags to Depth Only.

Culling mask first to nothing then only select newly created layer.

Set the Depth to 1 (you can have even more than 2 camera and arrange according to depth)

Set projection to Orthogonal (for your case only).

Select your main camera and just like the second camera, set clear flags to Depth Only but remove your new layer and keep other layers selected.