You are correct in that having two people both well-versed in both the system and the change they are making is a waste. However, that tends to be rare - the more common situation on a team with no newbies is that you will need to solve new problems with new methods, possible integrating with new systems.

If you have a team of experts, and you want to keep them all experts, it's much more economical to have two people learning the new code as it's being written. Theories about what might or might not be helpful have very little value until tried in the field.