Joe,
You'll get no disagreement from me that, in an ideal world, 2 pilots are better than one under high-stress situations. However, a couple of points need to be made.
Firstly, the post-war Canberra only had one pilot. All official heavy bombers after the Lanc did have pilot and co...but the Canberra didn't. I know, it's the exception that proves the rule. Nowadays we wouldn't consider such an aircraft without 2 pilots...but we'd also not have the rear-seat crew. You're right about all other postwar heavy bombers though.
Secondly, the Lanc could be fitted with a second set of controls. It was far from ideal and probably would have made things interesting for the bomb aimer accessing his position. However, such an installation could have been used (indeed was used operationally on occasion) if the need arose.
Thirdly, your comment about the flight engineer being ballast is a little unworthy. In reality, the flight engineer would come forward to occupy the dickie seat (essentially where a co-pilot would sit on most bombers) for take-off and landing to assist the pilot. While certainly not as capable as a fully-qualified pilot, I think it's a bit harsh to say he was "one step above ballast".
Bottom line for me is that the B-29 was clearly the better aircraft for the nuke missions. However, it's interesting to consider what options might have been on the table if the B-29 didn't become operational at the right time. I think the Lancaster, while not as well-suited, could have completed the mission, although it may have been a one-way trip for the crew. Then again, the Dambusters Raid didn't hold out much odds of survival. It was a different time and people volunteered for all sorts of crazy one-way missions. It's also worth considering that, apart from the Lanc, the Allies didn't have many other/better options for nuke delivery in 1945.
I think some of the arguments in this thread, on both sides, are being stretched just a little too far. This is my attempt to rein things back just a wee bit 'cos I've found the discussion fascinating.
ATB,
Mark