Several companies made en-bloc units that would either thread onto the driver, or drop onto the driver (depending on the age of your Sturmey AW). Those are first preference. It is possible to make a crude 2-cog unit with appropriately sized cogs set back-to-back on the driver. With that cruder set up, you need to play with the spacing to get a good chain line, but something that is doable.
The complications go beyond what is on your driver though. You need to decide whether to try to run a 1/8" chain (as one would with the 1/8 Cyclo set up), or whether you are converting everything over to 3/32" (and with that comes replacing all different manner of parts on the drivetrain). Hence why purchase of the hybrid set up as a unit (en-bloc cogs, Cyclo derailleur, shifters, etc.) was the popular way to go. It's something that is trickier to piece together than just finding a couple cogs and guessing at what goes where. It's doable but you need to be ready to take it on as a bit of a project.
