Rather than a dual acting piston, how about have two pistons directly opposed in the same bore with a central injector. A bit like a single cylinder from one of the Deltic units of Napier fame. British Rail made good use of these in the Class 55's.
These had two eighteen cylinder (thirty-six) piston engines on a common crankshaft with the generator in the middle.
72 pistons - on a two stroke.
http://rowla.dyndns.org/justin/img/piston_deltic320.mpg