The aircraft (Maia on the bottom, Mercury on top) were commissioned by the British government to fly transatlantic cargoes (mainly mail) in the late 30s. Built by Shorts in Belfast, Maia was a modified production model while Mercury was made for the job. The problem at the time was that an aircraft could just about get across the Atlantic but not with any useful payload. By using Maia to lift Mercury to operational height, not only was Maia's fuel demand reduced but it could be configured purely for efficient cruising. The experiment was successful and used in service but ended with the onset of WW2. Wartime design advances soon made the concept obsolete of course (thanks to comments from Tonyand03)!
strange that such a strikingly unusual image wouldn't stand out in my mind that I'd seen something similar before