k thanks for all the answers. i can build my B29 now.

A few things to consider: FIRST, as others have pointed out, B-29 wasn't designed to carry atomic bombs so they had to be retrofitted in order to carry such large weapons. Atomic B-29's were called Silverplate B-29's. Look them up to learn more.
SECOND, during WWII America could only make enough weaponized uranium/plutonium to build 3 or 4 atomic bombs per year and the cost was staggering.
THREE, the Little-Boy bomb was very simple to design and build, but it used a gigantic amount of uranium, much more than the US could sustainably produce. So the scientists developed a more efficient implosion bomb, the Fat-Man. The Fat-Man only needed 1/10 as much nuclear material to work, but it was extremely complicated and very difficult to design and build. In the end, Fat-Man ended up in mass production and only a few LittleBoy bombs were ever built. Also, when the Soviet Union stole the designs and built their own atom bombs, they were the Fat-Man type.

But as WolfSpark said, it could only carry one at a time. The Enola Gay targeted Hiroshima and dropped Little Boy. Three days later when Japan refused to Surrender, they sent the Bockscar after Yokohama (Tokyo) with Fat Man on board. They, however, found technical difficulties and dropped the bomb on Nagasaki instead

The bomb bay also had to be retrofitted. It couldn't carry them normally. The bomb bays were designed for conventional weaponry

Both Bombs were too heavy to be carried at the same time.
The Little boy and Fat Man could only be carried in the primary bombay, by which they were still way too big to fit both onboard.
You have to choose 1.

2 separate bombs by 2 separate b29s (enola gay and bockscar from memory). They could also carry conventional munitions, just Google it.

He Carried Little Boy Then Carried FatMan

Am confused