https://www.cxmagazine.com/gravel-bi...ost-found-2018

This intrigues me, for my own Surly Cross Check singlespeed (freewheel). Looks like an interesting experiment. Could be nifty to be able to gear down just a little like that, and just reposition the wheel in the sliding dropouts. What would I need to consider when mounting the extra smaller ring--I assume you need some kind of spacers? Obviously the chain line will suffer a little--how big a deal would that be?
Alternatively, could you mount two sprockets to the rear cog?

EDIT: just found this: https://wabicycles.com/collections/c...ixed-cog-18-16
Looking to see if there's a freewheel variety...