I've done it. It's pretty easy if you have any decent fabrication skills. I did mine without any welding.
The towing bike: Old Raleigh CX Dash (38c tires) with a Surly Nice Rack.
The towed: ~20lb rigid SS 29er
Remove the front wheel and lash it the the main triangle. I use velcro straps. The next part is making a mount. Most people make the mistake of trying to use a standard axle holder for a roof rake or something. You're making a trailer that happens to be a bike, it needs to be able to pivot. Use an old front hub instead. Now all three wheels can go up and down with the road. Next thing is mounting the hub. Mount the hub at approximately the same height as it would be if the bike had the normal front wheel on it. I used two chunks of flat stock and 4 p-clamps per side to mount the stock to the rack at ~40 degree angle downward to the rear. Then I put two full thread bolts through at the end that hangs past the rear wheel and used two U-bolts to secure the front hub to it. The last important element is a bungie cord that goes around the handlebars of the towed bike and around the rack or some element of the towing bike. This keeps the bike upright in sharp corners and adds stability to the entire system. Without it the towed bike will bounce and wobble a lot more. Also, when turning the rear wheel swings in and if it swings far enough, the fork can pivot down and drop, leading to you dragging a bike instead of towing it.