Some of the original 747 concepts (60s vintage) were twin deck, but if you're only transporting 350 people with those day's materials, there is a weight penalty to two decks compared to one. Remember that in those days the 747 was a big single decker with a hump. Also, the twin deck proposals could not have been used for cargo like the 747. Since Boeing expected the 747 to become a cargo hauler after SSTs took over, cargo was a big priority.
Amusingly, one of the 747 proposals ("The Anteater") had a single deck like the 741, but instead of the hump it had a lowered flight deck in best Beluga style. The objective, just like with the hump, was to ease loading of cargo.
In the early 70s, after the 747-100 was well established, Boeing looked at the following:
1. 527 pax baseline - plain 741 with lots of seats.
2. 716 pax - 300 inch plugs fore and aft of the wing. Upper deck unchanged.
3. 666 Pax - 60 inch plug fore and 140 inch aft of the wing. Upper deck stretched to the wing.
4. 847 pax - double deck to the fin, where the fuse comes down in a way similar to the SP
5. 1000 pax - 160 inch plug fore and 140 inch plug aft of the wing, double deck to the fin, where the fuse comes down in a way similar to the SP
Long-range (same bodies, other numbers of seats):
1. 385 - plain 741.
2. 544 pax - equivalent to 2. above.
3. 472 pax - equivalent to 3. above.
4. 624 pax - equivalent to 4. above.
5. 732 pax - equivalent to 5. above.
The oil crisis put paid to all these fun plans, but it's interesting to note how the A380 concept could have flown back in 1980 or so.
(Source - "Boeing 747 Design and Development Since 1969" by Guy Norris and Mark Wagner).
So it is definitely possible, but unlikely now. In the 70s, the 747 wing was state of the art. Nowadays it's rather dated. And if you're going to redo the wing, you've basically rebuilt the aircraft from the ground up anyway.
EDIT: The book has some interesting concept drawings of all this stuff, and pics of the 60s double-decker and anteater models. ISBN 0-7603-0280-4.
[Edited 2004-04-07 20:07:29]
[Edited 2004-04-07 20:15:01]
[Edited 2004-04-07 20:19:20]
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo