Thanks for the Shimano 600 suggestion, is that the narrowest cassette available? Ie, with the lowest dish?

It's not just the number of gears though, it's the gear spacing. As pointed out, it's not obvious that I will be able to accomplish two cogs on same side with standard parts but I see a few theoretical benefits to this setup that I'd like to test out:

- Half the pulley drag, especially with road debris on
- Better chain angle
- Stronger rear wheel
- Simpler/lighter overall transmission