It happens all the time. A client wants to commission a portrait of multiple family members, but is stumped by how exactly to go about the composition. She likes the idea of having one work of art which includes everyone, but worries about who will inherit the painting down the road.
This was the case with a mother who commissioned a portrait of her ten year-old twins. The children were always playing together in real life, so she naturally wanted to capture that in their portrait. We decided on the above/below painting(s) that would be equally lovely hung as one or as separate works of art.