One other possibility just brainstorming,another possibility comes to mind. It won't be very realistic if your goal is to look like prototype standard/narrow gauge dual trackage, which would have 3 rails, though the inner rail would not be centered as it is on O gauge 3 rail track, then my idea won't work. You could do dual gauge by having 4 rails, with the HOn3 track centered around the middle rail. Obviously, this would cause problems with switches, but given that the middle rail to outer rail is not exact for HO gauge (.625 is O gauge 3 rail spacing between inner/middle rails, .65 is HO gauge track), you might have problems with binding and other issues with the HO gauge wheelsets using the outside and middle rail.
The other problem with using the middle rail is the obvious one, HOn3 uses DC. You could put a rectifier in the HOn3 engines, you also would need to be careful with speed control given that AC transformers put out a max of 18v, 12V DC for scale trains. If you wanted to run command control using the shared third rail using DCC I would hazard a guess is possible, but likely would be complicated, too. a 4 rail approach would allow basically isolated power, but allow dual running. The tough part of 4 rail would be switches, not sure if that is even possible (Haven't tried sketching it).
In any event, I suspect any implementation would require a lot of trial and error and custom fitting. If someone did this commercially, there may be existing track out there at the very least to act as a pattern.