I will get Texture2D from Rendertexture. The RenderTexture is Target Texture from seconde Camera.
GameObject com = GameObject.Find("Camera2");
RenderTexture myRenderTexture = com.renderer.material.mainTexture;
byte bytes = myTexture.EncodeToPNG();
File.WriteAllBytes(Application.dataPath + "/../SavedView.png", bytes);
Also check out Application.CaptureScreenShot. It won't capture the second camera, but I'm not sure what or how you are using the render texture so you may just want to run the game in the editor if you want a specific image and capture the screen.