Two cameras on rendertexture, one with depth of field

Hello,

I have a scene with some objects and I want to apply depth of field to them. But there is also a background plane which shouldn't be affected by it.
So I have one camera for the objects with "Clear Flags: depth only" and another camera for the background plane. Both render on one rendertexture:

Code (csharp):

1.  
2. rt = new RenderTexture(1920, 1080, 24);
3. rt.Create();
4. sourceCam1.targetTexture = rt;
5. sourceCam2.targetTexture = rt;
6. var newMat = renderer.material;
7. newMat.SetTexture("_MainTex", rt);
8.  

Everything renders fine without the dof effect. When I turn it on, I can see only the front camera rendering with it's black background (which should be transparent to see the second camera image).

Any ideas?

Thank you!