Question: As the Carson City Mint used two letters, CC, for its mintmark, why didn't the New Orleans Mint use two letters also -- NO?
Answer: In 1838 when the New Orleans Mint began operations the O mintmark was sufficient and was not confused with any other mintmark (the only other two at the time were C for Charlotte and D for Dahlonega). However, in 1870 when the Carson City Mint started, probably someone at the Treasury Department thought that a single letter C might be confused with the C used earlier for Charlotte (where the United States had a branch mint from 1838 to 1861).
However, that was not the case in 1906 when the Denver Mint opened, the Treasury reused the D mintmark that had been used decades earlier on coins struck at the Dahlonega Mint.
So, it seems there was no consistency when making this decision.