Double deck VC10
I too am amazed at this, but I'm pretty sure the Deux Ponts' two decks long preceded this project, as the DP was developed in the second half of the 1940s, whereas the VC10 was developed in the second half of the 1950s.
So the A380 might not have been the world's first full two-decker jetliner and it certainly isn't the world's first two deck airliner-that honour belongs to the Deux Ponts, it would appear.