I cannot comment, but I just wanted to share some information (as I just wrote a paper on Project Apollo). First, there is an excellent resource available from NASA called Chariots for Apollo. The chapter relevant to the question is Chapter 1- Concept to Challenge.
Hobbes' quote is correct, historically, that studies had been undertaken by NACA since the mid 1950s. Pearson is also partially correct that NASA wanted the workload split (for redundancy and other reasons). Project Mercury and Gemini were both adapted to the needs of Apollo. However, another great resource, though not official as per the page, is Guidelines for Advanced Manned Space Vehicle Program. You will want to pay special attention paper written by Stanley C. White Human-Factors Considerations.
NASA decided on two astronauts for many reasons, with only some given by Hobbes and Pearson. Hopefully, the provided resources will help explain some other reasons aside from the work and abilities of the astronauts.
Personally, and without justification, I feel NASA chose multiple astronauts because they did not understand how extended and stressful missions in space might affect a human. I believe this is somewhat shown by the following example. NASA quarantined all Apollo astronauts for 21 days, up until they realized there was no chance for them to bring back anything dangerous.
I also feel their answers ignore the political ramifications behind Apollo (NASA was pressured to meet the decade goal), but that is a topic for another day.
In response to the mass penalty, from the provided resources you can see that NACA and subsequently NASA originally prepared for a mission with far more available resources. Mass in the preparatory stages was not considered to be an issue. The Goett Committee fully expected a two week long mission (against the 8 day Apollo 11 mission).