I used simple centrifical clutches. The back engine had the dual sprocket clutch on it.ties the two together. They wont engage until the rpms are high enough. The throttle was tricky, getting them both to rev at the same time. Its not exactly perfect Im sure. I pull started each one individually. I built this one just for parades. After the hometown parade, I got stopped by a local cop. He had to get a photo with him on it. It was pretty cool.