What are the current Co-axial aircraft and how does their propulsion & gear system works?

Answer 1

Adam A500. It has two six-cylinder piston engines, a pusher and a tractor. [http://adamaircraft.com/specifications.asp] The engines are connected by the fuselage, but the props are not geared together or anything. Or, are you talking about, say, counter-rotating props, like on the Reno P-51, Precious Metal? [http://www.mustangsmustangs.com/p-51/survivors/pages/87-4488.shtml]