Some multi-engined tractors run the motors hooked crank to crank. They have a coupler bolted in place of the flywheel that is splined and a coupler that fits in place of the damper pulley that is splined, and they slide together. Kinda hard on the crank that has to take the power of both motors! On the newer built tractors, all the engines are mounted separately and have their own clutch and are bolted to a crossbox, that transmits all the different engines power to the single planetary rearend.
Here's a link for you. Scroll down a little for some pics.
http://www.tedderpullingperformance.com/