Cheap singlespeed cyclocross conversion

I dont think a double up front and a double at the back would be not sensible, why not have gears?

You do have gears. Two.

Chainline would be a bit messy

The chainline would be perfect, better than with a derailleur.

...and I think chain length changes like that I think it would be hard to get the same chain in those dropouts.

The axle doesn't move because you've carefully chosen 2 gears with the same chain length.

The Double/Double is a unique drive train system the we offer. The system is comprised of a double chainring machined from one piece of aluminum, mated with an ENO crank and a DOS ENO freewheel. The configuration for a 26" wheel bike is offered with a 38/35 big ring mated with a 16/19 freewheel. The 29er configuration mates a 31/34 ring with a 16/19 freewheel. The system works by either running the chain in the outer tooth positions or, alternatively, in the inner tooth positions front to back. The gear ratios are significantly changed when switching from one combination to another, however, the chain length does not need to be altered to accommodate the two differing ratios. Running the ring in the outer position tends to be more suitable for on-road style riding, whereas, riding in the inner ring position lends itself more to off-road riding.
From here.