How to prevent trash bins from falling by placing apart or side-by-side, on a windy day?
In my area the problem with the sloped driveway is solved by simply placing the bins along the curbing to the side of the driveway. The roadway surface is typically sloped down a small amount toward the curb and this helps to keep the wheels of the bin squarely against the curb. This in my experience offers more support for the bin in high wind conditions by trying to place them directly adjacent with each other. 

The negative factor with placing bins directly together is that the trash company driver will often bypass pickup at your location if you do this. And you can expect to get the bright orange warning sheet taped to your bin if you persist in doing this.