That would be good, very good I think. The only issue I see with doing that is with the space I have between the track to the north of the platform and the inner track at the top of the layout, as it is, I only have about 10" of space there.
IF I could move the "inner single track" to the north of the main line then what you propose would be doable AND look great. Unless I increase the grade on the main line up there, and increase it fairly significantly, I don't have the clearance for the inner track to pass under the main line. Alternatively, I could (maybe) have the inner single track passing "over" the main line (which is what I would prefer to be honest) but again, I would have to change the grades on both lines fairly significantly.
At the moment my grades are 2% - 2.3%. Some people claim you can go as high as 4% with N Scale and that 3% isn't an issue. I am a little sceptical of those claims; although, if they are accurate, then making the changes may well be possible.