Cycles shadows only / reflections


im quite new to blender and therefore have tried already a lot to solve my problem, but nothing was really convienient.
I´m trying to render a couch, in different render layers so i could change the background later to any colour i need. Thats why i rendered the couch in one layer and the shadows in another layer. so far everything works fine but the shadows looks quite noisy… after putting the bilateral blur its okayish. the main problem in the end is that the shadows look extremely flat compared to the rendering where i render the couch witch the background. This is probably because of the reflections etc. of the plane in the shadows, so is there any chance to render also this in another layer or in the shadows layer?
thanks !!!

This one is the rendering with the background rendered together!

I think this is a engine limitation. Cycles render noisy shadows in image output. To fix that enable Denoising Data at your View Layers Properties, them Denoise the alpha channel and use as a fac input to a Color Mix Node and set to Multiply, first color will be your background (also works using images). Denoising the actual shadow RGBA pass doesn’t work.

I also had a similar issue with quality using shadow catcher: Shadow Precision Issue - Cycles