Does anyone know which brother is John and which is William Hanover?

Gerald Hanover has sent a photograph of his  grandfather John Hanover and John’s  brother William.  Both of them feature in Barrow Hill’s book of the First World War.  John Hanover was a driver with the Royal Engineers, his brother William served with The Kings Royal Rifle Corps.  Both brothers served abroad throughout the whole war. Unfortunately Gerald does not know which of the two is his grandfather and wonders if anyone could help with information.