By the 1940s and '50s, parallel twin motors were by no means new technology. In fact, the first motorcycle to run this configuration was a Hildebrand & Wolfmuller motorcycle way back in 1894; it was driven by steam and is regarded as the world's first series production motorcycle. https://www.hagerty.com/media/motorcycles/stock-stories-1949-matchless-g9-and-ajs-model-20/
My Father who was a young man in Belgium during and after the 2nd WW , said he rode a FINE Matchless for a few years.. somewhere this house is a photo of him standing proudly with this machine. Thanx for the read.