I just configured my twinview setup to rotate the display for the monitor on the right. It is working well rotated, except for one thing. My subpixel font rendering is configured to use RGB, which is correct for the non-rotated monitor, but with the other one rotated, it should now be VRGB.
Since this setting is only available for the display as a whole, it appears there is no way to have a different subpixel font on one display as opposed to the other.
Is it even remotely possible that this can one day be done?