Unity - Ambient Occlusion artefacts

Question

When I create two cubes from Unity > Component menu. And turn on AO the bottom cube is not shaded properly. I mean the upper cube is alright but the bottom cube does not have any shadow present. What am I doing wrong ?

Lighting setup:

Result:

Answer 1

Reduce Indirect Resolution under Baked GI. Bake with these setup.

Modified upon your setup:

Result:

Also close the corners if possible.