2 Camera-Setup overrides Post-Processing

I have a scene, rendered by camera #1, which has post-processing effects on it. On top of it I render my UGUI. Due to limitations with Particles in the GUI my GUI is rendered in "Screen-Space Camera"-mode by a second camera (no post-processing, "Depth only"-flag). However, this completely overrides all post-processing effects applied to the first camera. Seems as if Unity's taking the first camera's rendered image before post-processing, putting the second camera's rendered image on top of it and then only applying the second camera's post-processing to the final image. Question: How to do post-processing on one camera while rendering a "Screen Space Camera"-GUI on top of it.