[Madison]

What's shown in that illustration is not a 'double-sprocket', but two 21 tooth sprockets placed back to back. This arrangement was suggested for the 2005 Kitbot as an easy way of implementing four-wheel drive, but results in the wheels at one end being offset from those at the other. The picture Karthik linked to is effectively the same thing. The offset will vary with the length through bore of each sprocket.

The difficulty in customizing the spacing of chain on the provided transmissions comes simply from the limited length of the custom-made output shaft. The arrangement shown in the first illustration will probably do the best at getting the wheels as close to flush with one another as is possible without facing off some of one sprocket.

I don't think you'll see much benefit in placing one set of wheels closer together over an another, as we probably spend equal amounts of time going in each direction throughout the course of a match.