matthewpiano:Surely what you are suggesting would not aid ventilation but inhibit it??
Yeah, in all likelihood it would... I was thinking from a shielding point of view (skim read original question and forgot about the ventilation bit when answering)... most seperates, as we know, have the vents in the top and this is why I said that I did not think that it was a good idea... there are hi-fi shelves on legs, but the likelihood of those shelves fitting into an existing shelving unit is slim..
Bottom line is that I think that it would be better to keep to the one unit per shelf plan and if you have outgrown your current shelving set up, replace it.