Here is the way I see it. Both pistons move outwards at the same time and reach top dead center at same time. This makes for better balance and is the only way you are going to get a pumping action to pull in air and gas. If both pistons went the same direction on one throw there would be no vacume to pull air & gas into crankcase. The air would be just shifting from side to side. Also the balance would be way off with 2 pistons stopping to left side of engine then back to right side. I say both pistons have to travel out-wards to top dead center...but maybe a better way to join the 2 cranks may be had. Capt,n.... Edit added Photo.