Well, there are the various Wagner twins, of course.'nother thought I had was overlapping the props so the thrust centerlines are closer to the centerline of the aircraft. I was looking at a Beech Starship when I thought of that. Extend one of the prop shafts and the distance between the prop centerlines can be reduced about 40%.
You can just make out the spacer on the left engine that gets the prop a bit further forward than the one on the right.
Haven't heard any sort of aeronautical analysis of this system, but suspect the overlap area has some horrid effects.
Wagner also just strapped two fuselages together, too....