Boy that brought back a memory! One of the modelers at the old Pacific Beach Model Railroad Club, which was located for years at the San Diego County (Del Mar) Fairgrounds, had a passenger train of two E6 A units and about eight streamlined passenger cars. The cars and engines had light gray/silver roofs and on the one side they were painted Southern Pacific Daylight colors and on the other UP Armour Yellow. I don't remember how he handled the nose treatment on the diesels but it didn't look like two paint schemes head on. Since the layout was designed to be viewed with trains popping out of a tunnel at one end and going into a tunnel on the other it gave the distinct impression that there were two separate passenger trains when only one was running. Very effective use of video special effects.
Now if we could have a red side and a green side to the coaches.......