Complex sentence: The first two B.E. aircraft were flown within a month of each other and had the same basic design, the work of Geoffrey de Havilland, who was at the time both the chief designer and the test pilot at the Balloon Factory.
Split into simpler sentences: The first two B.E. aircraft were flown within a month of each other and were of the same fundamental design. They were designed by Geoffrey de Havilland, who was at the time both the chief designer and the test pilot at the Balloon Factory.