There was a system years ago that attached the front forks of a second bike to the rear axle of a first bike. It was touted as a cheap way to have a tandem. Can you open the rear quick release on you road bike far enough to fit the front forks of your MTB? There are special trailer units for towing canoes and kayaks behind bikes. Try adapting one of them. Or just break down and buy a normal bike trailer. Then bungee your MTb to the trailer.