Edgar T Westbury published a design back in 1950 for a 10cc, twin cylinder, four-stroke, side-valve petrol engine that he dubbed the Seagull. The engine evolved from the earlier 15cc four cylinder, side-valve Seal. The basic design was also adapted to a single cylinder, air cooled variant of the engine which engine became known as the Seamew. Plans for the Seagull “the classic launch engine” are still available from https://www.sarikhobbies.com/product/seagull-pe25/

The pistons move in unison so that the "rocking couple" caused by having the pistons move anti-phase is eliminated. So instead of shaking and rocking, it just shakes. The advantage is one cylinder can be on the compression stroke while the other is on the exhaust stroke. This evens out the firing pulses to one every 360 degrees and allows a lighter flywheel to be used and to dispense with a distributor. It fires both plugs at the same time every cycle; the spark in the cylinder that is exhausting will do no harm and the engine is significantly simplified. A crankcase breather is fitted with a simple flutter valve.

This example by B Bedder was shown at a Midlands Model Engineering Exhibition.