Question Transparent shader with ognoring target RenderTexture content

Hi.
I have two transparent RenderTexture: background and drawing. Shader should merge this two textures to transparent Quad.

On event I need to copy drawing texture to background texture. For this I create new background2 texture

Code (CSharp):

1.  
2. layerCopyMaterial.SetTexture("_background", background);
3. layerCopyMaterial.SetTexture("_drawing", drawing);
4. Graphics.Blit(emptyTexture, background2, layerCopyMaterial);
5. background=background2;
6. Graphics.Blit(emptyTexture, drawing);
7.  

After this Quad should have same image. But I found problem that initial color of background2 texture is affecting merging result.
If I make shader non blending then quad became non transparent so this is not solution.
I use blending like
Blend SrcAlpha OneMinusSrcAlpha

Is there any solution for this? Only to use two shaders with and without blending?