I have just bought Dynaudio MB6As. My previous set, Monitor Ones Mk2, sit over them.
From now, 90% of time I will be working with the Bm6As, so these are the ones to be at correct ear level.
I imagine you might choose your best set and keep them at the proper height.
Putting these monitors one closer to the other would generate lots of troubles, like stereo image shift, reflections. I
I always try to keep monitors at least 80 centimeters from every surface (side walls, back walls, racks, windows).