Yop

The post is a little old, but i guess it could help:
VeloSolo Twin Cogs - Dingle?
double fixed cog on a disc hub, if you're good with a press drill you can do it yourself for ~12€ (without the price of all cobalt bits you will burn if you don't lubricate). It's safe, cheap, quick to change for an other cog. And to keep your chain tension, i think the better is to keep a double crankset, and make 2 ratio (like 39-20 and 42-17). You haven't to change your BB, cool no ?