Could someone please explain the physics as to why two trash-bins could be more stable if placed together instead of apart during windy conditions? I recently asked the following question on another StackExchange community(Home Improvement: Preventing trash bins from falling by placing apart of side-by-side, on a windy day?), but I would like someone to explain why placing two garbage bins together could be more stable together than if they were to be placed apart during windy weather.  As I understand, the aforementioned is supported by boundary conditions?  In any case, I would appreciate an explanation.  Thanks!
 A: Looking at the problem apart from the "sloped driveway" aspect in your previous question...
The rationale of putting the garbage bags side by side is similar to using an "I" beam for structural support.  The wind force only acts directly on the first bag.  The second bag has force from the first bag, but it's cut in half, since they end up sharing the wind force.  Logically, this only works if the wind is in the same direction the bags are aligned.  
In case you're wondering, if you're doing more than one row deep, it's best to stagger them so that the rows can "interlock" somewhat (do a search for "hexagonal packing").  Anecdotally, I typically stack my leaf bags 2 rows deep in the fall for this reason, since the paper bags are inherently less stable to begin with.
A: First one bin toppling because the torque about $X$ due to wind is greater than the torque about X due to the weight of the bin $W$.

It depends from which direction the wind is blowing.  If both bins are facing the wind I do not think it makes any difference if there is one bin or two.
However if only one bin presents itself to the wind and the other is behind then the result is different.
The bin at the back has little horizontal force acting on it due to the wind.  
The front bin does have a horizontal force on it due to the wind but the right hand bin exerts a force on the left hand bin which produces an anticlockwise moment about X on the left hand bin to reduce the chances of it toppling.

On second thoughts it might be more realistic if the rh bin was also at a slight angle?  The point is still that the wind must provide more torque to topple the bins.
