It's not specified in the British recommended standard either (which I personally think leads to a more comfortable console than the AGO spec).
https://www.organworks.co.uk/brit_console_dimensions.pdfI guess the reason it's not specified anywhere is because it depends on the width of your particular pedals.
You need them to be far enough apart that you can operate one pedal on its own when required, but close enough together that you can operate two pedals simultaneously with one foot when required.
In the case of my console, the gap between the swell pedals is about 1 cm.
Graham