What are the advantages and disadvantages of a double bubble fuselage?
Double bubble is the lightest way to make a pressurized fuselage taller than it is wide.  It maintains the pressure hull elements, skins and formers, closest to pure tension, like a balloon, so they can be a bit lighter since they see little bending stress from pressure (only the interconnecting floor beams see significant bending loads, from the weight of the payload).
The A380 went with an ovalized cross section because it provides a better packaging compromise when you want two decks of roughly the same floor area, although it would be somewhat heavier than a DB cross section because the frame elements of the partially flattened sides are subject to a certain amount of bending because the pressure loads are trying to circular-ize the oval (the floor beams help but the oval sections away from the beams are still subject to that stress).
Think about what would happen if you really cranked up the pressure inside the A380, and imagine how the pressure hull would deform under stress; it would swell out to a more circular shape except where retained by the floor beams, before going &quot;POP&quot; (you'd end up with a kind of &quot;triple bubble&quot; before it went bang).  That deformation load trying to make the oval a circle has to be absorbed as bending loads in the frames, so they have to be heavier than they would be if they were already purely circular.
The packaging advantages over a pure double bubble make it worth the weight penalty.
The original double bubble pressure hull was the Curtiss C-46, which was originally designed as a pressurized airliner (otherwise they would have used an oval cross section), and used the configuration to make a pressure hull that was a light as possible while keeping it relatively narrow.  Then you had the Boeing 377 Stratocruiser, which also went that way in order to graft a passenger cabin on top of a B-29 fuselage and wing without making it too heavy.