Are you planning on running down the middle, or use one outside rails for both gauges? I assume you want to use a common outside rail.
I remember trying this as a kid with my o gauge trains and my brothers' ho stuff. As stated above, the scale is really close but doesn't quite work out, my brother's trains would always get lopsided and start falling between the rails.
If you are will to relocate the center rail however, I think the small offset needed to make your ho stuff work would not bother the wide pick up rollers on most 3 rail equipment. I am not away of commercially available track to do what you want, but I think you are headed in the right direction talking about a thin center track, but if you need to move the center rail I think you are much better off with mth scaletrax or atlas O. The reason I say this is that Gargraves and super O both have a center rail that is embedded in the tie. Otherwise I think you are totally into custom track from the get go.
Check out this thread for some views of how the various track systems are constructed.
My plan is to put each HO/ON30 Rail in between the center and outside rail, as I figured that would be the easiest thing to do, and it would allow me to minimize the modification needed to the track, but like I said, my biggest worry if I do this would be clearance for the underside of either locomotive. if the Narrow Gauge rails are too low, than the ON30 engine won't sit correctly on the rails. if the rails are too high, then my O Gauge Engine won't sit on the rails properly.