If you were to compare the B29 and the Lanc, you would find the B29 was limited by having two relatively small bomb bays, whilst the Lanc had one large one. And the small bomb bays ther B29 had where further reduced in usefulness by having a pressurised tunnel running through them, so that the crew could move about the plane's pressurised areas.
The lanc could carry really useful large bombs, but the B29 had to carry lots of smaller , less effective, bombs.
No good for Bunker busting, U boat Pens, Eathquake bombs or the like.
The B29 could only carry the Atomic Bomb after it had been modified to link the two Bomb bays. For this reason the Lancaster was initially carefully concidered as the delivery system for America's "Big Bomb" but eventually it was ruled out( American indepenance and pride rather than an unbiased concidered judgement??).
The Lab was capable of further development, into the Lincoln and ultimatley the Shackleton.
To my mind this makes the Lanc a far better airplane!
I have never seen anything about the B29 carrying Two Grand Slams. The Literature I have indicates its maximum bomb load was only 9072 Kg or 20,000 Lbs.
Where does your info about this come from Steve?