The bomb that was tested during Trinity was a plutonium, implosion device, completely different from the uranium, gun-type device dropped on Hiroshima. The designers were so confident of the latter that it was never tested before being deployed.
It probably would have been more efficient to dilute the U-235 into reactor grade and use it to manufacture weapons grade Plutonium. However at the time the U.S. only had two bombs (the ones dropped on Hiroshima and Nagasaki) and it was probably judged politically expedient to drop two instead of one.
All of these processes were brand new at the time and mostly untested. Plutonium enrichment at the Hanford site used a reactor design that was validated well into the construction of the site. The Manhattan project already had a lot of risks, and they weren't going to attempt chaining projects for something unproven. Real experiments came after the war.