Setting camera ClearFlags from OnPreRender erases the previous render?

I am trying to render twice, once to generate the skybox, and again to render the foreground objects.

Currently I must use two cameras to properly display a falloff shader I created (with help from Bgolus) for my floor, I use one camera for rendering the skybox and another for the rest of the scene.

But now I have another camera in my scene that instead uses a renderTexture, so i can not think of a way to use two cameras for this.

I am having trouble with this code because even though I render twice and it would seem to be easy, instead once I change the clearFlags I lose the previous texture and my skybox goes black:

Code (csharp):

1. void OnPreRender()
2. {
3. //First render the skybox background texture:
4.    m_camera.clearFlags = CameraClearFlags.Skybox; //only render skybox
5.  
6.    m_camera.cullingMask = cullingMask1; //Layers: Nothing - for skybox rendering only
7.  
8.    m_camera.Render(); //render the skybox background
9.  
10. //Now render the foreground and place on top of the last (skybox) texture:
11.    m_camera.clearFlags = CameraClearFlags.Nothing; //This is where my skybox render gets wiped to solid black.
12.  
13.    m_camera.cullingMask = cullingMask2; //Layers: Default, Water, Others..
14.  
15.    //m_camera.Render(); //will render on it's own after OnPreRender
16. }

Is it possible to preserve the skybox render and render a second texture overtop of it?

Sorry for my inexperience... Would commandbuffers be a better alternative?

Thank you very much!!