You're confusing things here. Just for general knowledge's sake, it would help to read easymamecab to understand why the display is essentially the same between 15khz/60hz and 31khz/120hz. Now, to the real question of using 31khz/60hz modes of arcade resolutions at double-scan: as I said before, it's not worth the effort, as using D3D is far easier and then you can, at least somewhat decently, have scanlines through the use of effects. Further, I see no reason to do arcade resolutions at 31khz/60hz double-scanned, as you can get the same effect in D3D setting bilinear filtering to 'off'. Try it, you'll see.