Great Portland Street seems to have been a hotbed of motorcycling in the 1910's. In number 173 was Rey's, where you could buy a Royal Enfield. In 208 was Godfrey's, in 100-136 was Maude's (also sold Enfields). Today 173 is a pharmacy and there are no motorcycle shops in the street anymore. Perhaps as a tribute, parking in front of it is for motorcycles only.
His grandson becomes acquainted with a motorcycle on a Harley-Davidson. He should have chosen a Royal Enfield. From "The Motor Cycle" June 15th 1916. Interesting comment about motorcycling in India at that time. And the caption probably meant the grandson of the Maharaja Gaekwad of Baroda.
David Blasco at royalenfields.com has a post about "Royal Enfield sidecar ideas you never thought of". And although indeed I had never thought of the idea of a dual sidecar, someone else had. In fact the article claims that as early as 1903 there was an article about one in The Motor Cycle. This particular one was attached to a Royal Enfield 6hp, for which as we all know, nothing was impossible. Alas, the article claims that having four wheels means the vehicle will be taxed as a car, which detracted from one of the main advantages of a sidecar rig, at least at that time in the UK.