Ok, we discussed this with Brass.
is still valid.
The circuit there will only support looking at one half of the Amiga screen at a time, it does not accelerate the colour signals.
The syncdoubler circuit on aminet has a multivibrator tuned to the VGA frequency, then it is either trigged by the RGB signals on the beginning of the line or by another multivibrator that is tuned to vibrate every half an Amiga line.
Thus you can select whether you get the first half of each line on the VGA screen or the second half. Meaning you get either the left or the right side of the Amiga's screen at a time, stretched to fullscreen.