[whpalmer4]

I've read the MMM many times, thanks. I'd say that if you don't have a substantial change in the duration when you double the resources on a task, it probably means you haven't broken that task down sufficiently or you didn't estimate it properly. Your initial estimate of how long it was going to take should have been longer to account for all of the extra work your co-worker was going to add to the schedule by pointing out things you need to do better.
Similarly, if the OP is saying "this block is going to take 3 weeks" and that number doesn't change as he changes the number of people assigned, my question would be "why bother assigning the extra people if it isn't changing the duration?" Well, I guess it makes sense if the extra people being added are managers :-)