I suspect that the sideways orientation is motivated by cooling. The mac pro sucks air in the bottom and shoots it out the top, so if you stack them, the top ma

I suspect that the sideways orientation is motivated by cooling. The mac pro sucks air in the bottom and shoots it out the top, so if you stack them, the top machine is sucking in the bottom machine's hot air. In this orientation, the whole rack is sucking air in from one exposed side and pushing it out the other exposed side; much easier to engineer around.