How to add an object after the background has been rendered ?

For an interior scene, I would like to determine the cabinet (size, color…) later.
I render the room and save it as a render layer 1 (At this time, I don’t know what the cabinet will be.)
After layer 1 completed, I render cabinet in render layer 2.
I try to use compositing to combine render layer 1 and layer 2, however, it really fake.
The problem is the wall near the cabinet didn’t reflect it.
One possible solution is separate the wall near the cabinet and move it to render layer 2.
However, to maintain such object (the wall) is quite diffcult.
Could you tell me how to fix this problem ?
Thank you very much!

Do each render to the MultiLayer OpenEXR file-format, and, in your RenderLayer settings, be sure to capture Alpha and Z. “RGBA” should be the default, but be sure.

This will give you the two pieces of information that you generally need: Alpha (“transparency” … where an object is and isn’t), and “Z” (distance from camera). Beyond that, welcome to the world of node-based compositing.

Shadows and reflections are difficult. Sometimes you must put “shadow-casters” into one shot, strictly to capture the shadows that will fall on something that isn’t there yet. You need the “shadows will fall here” data.