Side loading is also known as double loading in art terminology.
It refers to an old painting technique of loading a brush with two colours next to each other. In the past it has been used for tole and other decorative painting on various media. The colours then create a gradation that blends the colours together in brush strokes.
Some artists use it actively. However, it is best applied in folk art, and in some calligraphy by which the necessary precision skills are being acquired through the production of multiple copies.