the actual snes output is 512 pixels wide. in hires-X and hires-XY mode a bg layer is displayed on every single one of the 512 pixels. in normal mode the bg later is displayed with every pixel doubled (effectively horizontal res of 256). in pseudohires mode, two bg layers are displayed at once: one is displayed on every odd pixel of the 512, and one on every even pixel. The example image etabeta showed above (at http://mamedev.emulab.it/etabeta/fast/files/0051.png
) is correct (though the exact 'order' of the interleave of the two bg layers might be backwards, this is difficult but not impossible to check on real hardware). be sure to click on it so you can see it in full res.
Hires-Y mode and Hires-XY mode on the snes are flickery because they're swapping the Y lines every 60fps field, for a 30fps full framerate. Or this is how it's supposed to have worked.