I take my idea from the old 2-stroke Deltic marine engine.... if you where to take a 12-cylinder boxer engine split it down the middle and give each bank of cylinders their own crank then face the the cylinder twords each other, of course then you would need an open valve coupler to join the banks and give an area for the intake and exhause ports.
you could still maintain the same displacement, compression ratio, and stroke with half of the cylinders, the combustion of the fuel could push down on pistons on each side.
you could also(in the manner in what i can see) double the ammount of intake and exhause ports, 4-intake and 4-exhaust.