That got me to thinking.  Long ago in a mill far away, my assignment as a project engineer was to install two defoamer tanks.  They were relatively small, about 15,000 gallons each as I remember.  They needed to be alongside a railroad track.  There was not much real estate available in this prime location.  

The pulp mill gave me a small spot, and I designed the tanks to be on top of one another.  I had a structural engineer do the foundations and I required the tank supplier to give me a PE stamped drawing of the design before they fabricated the tanks. 

A funny thing happened.  The base was not any thicker and did not carry any more rebar than it would have if I had done the tanks separately, that is only one tank on the base.  Additionally, the tank walls on the bottom tank were no thicker than they would have been if a second tank had not been on top.  And, I saved the cost of one cover. 

Is this not a design that fits our Light Green Machine philosophy?  Where else can we apply this?