This model dates to the early days of N scale (late 60's / early 70's). It was primarily imported by Minitrix / Model Power. AFAIK, they all came with two containers.
I don't know what specific prototype this model might be based on (if any), but generally speaking, flat cars like these first entered service in the 1960's. They were designed to accommodate the latest 40' trailers that were just being introduced on the nation's highways. Each flat car was set up to transport two of the trailers. Subsequently, these dedicated TOFC (Trailer On Flat Car) flatcars became the defacto standard for intermodal service during this era.