side by side can have a literal meaning, for example when two people are sitting next to each other on a sofa watching television. In addition, it can have a metaphorical meaning, where a group of people are engaged in a common endeavour.
As this NGram graph shows, the expressions fought side by side and worked side by side are common, but when you look at actual references, it is clear that it is not a literal meaning. Here is a typical example:
The 167th and 168th Infantries were brigaded together and fought side by side throughout many campaigns on the western front. - Journal of the Senate of the General Assembly of the ..., Volume 48, Part 1939
It is very unlikely indeed that two regiments (several thousand men) stood shoulder to shoulder in a second world war combat situation.
I think that, in this context, painting side by side is a metaphorical expression: the two artists were engaged in a common endeavour.