"In pre-1962 Missals in the Ritus servandus in celebratione Missae, X, 6, this Confiteor is stipulated. In the same section in the 1962 Missal it simply is not mentioned, but nowhere in the rubrics is it forbidden in an explicit manner. Apart from this omission the ordinary of the Mass was not changed."
The Missal of 1962 - A Rock of Stability
by Michael Davies - Spring 2001
go to: http://www.latinmassmagazine.com/articles/articles_2001_SP_Davies.html
Additionally, Klaus Gamber, a favorite liturgical author of Pope Benedict XVI, wrote in his critique of the liturgical reforms that the second Confiteor was a legitimate option in both the 1962 and 1965 versions of the Missal, even though they were not specifically mentioned.
Regarding the question of the second Confiteor in the 1962 Missale Romanum, in the Ponticale Romanum (Marietti, editio iuxta typicam, 1962) there is a second Confiteor prescribed in Ordination Masses. Thus, the use of the second Confiteor is not foreign to the use of the 1962 Missale Romanum.