Or you could do like Tommy Ivo did, and place the engines side-by-side. If I recall, he doubled up on the flywheel ring gears and meshed them together to connect the engines. The clutch and driveshaft were on the left-hand one, which matched up with the offset in the rear axle housing.
Way back in the 30's or maybe 40's, somebody (Auto Union?) took two straight 8's and joined them by gearing their flywheels together. They called it a U-16 arrangement, and it made a ton of power, but was too peaky to be controllable.
Twin 6's would be way cool, imho. Especially if you could find a second engine that mirrored the first one - have both sets of exhaust on the outside of the whole thing.