A twin simple has very smooth output, there are no deadspots of torque as there is with a single or double expansion compound, I ran my 3 X 4 twin with no flywheel for a long time and then added a flywheel for no other reason than peer pressure , there was very little difference afterwards. It's only real purpose is a pulley to drive the generator. Car steam engines didn't use flywheels.
Outboard motor shear pins float in a slot and many of them are 1 and 2 cylinder. The key, no pun intended is to size it appropriately.
-Ron