Hmm, from some poking around, it looks like it might have been possible to make a good chunk of the 377's lower level into a second passenger cabin at the expense of a large part of the cargo hold if someone had an operational requirement for it, but I don't think anyone tried developing the airframe in that direction.
Another possibility from the immediate post-war era could have been the Lockheed R6V Constitution, which first flew in 1946, though it was considered a failure- the USN felt it was too expensive to operate, while Pan Am ultimately decided it was too big & underpowered to be practical even though the design had originally been proposed to them as something that would exceed the performance & capacity of the Constellation, effectively killing the proposed airliner version; the 2 prototypes in USN service also turned out to have too short of a range & prone to engine overheating; a turboprop motor being developed by Wright might have salvaged the design but never got outside the prototype stage