That rule only applies outside the paint. Centers can keep two hands on their man at all times.
That's not true! They can use a forearm, however.
Rule 10: Page 89
"Section 1. Personal Fouls
Art. 1. A player shall not hold, displace, push, charge, trip or impede theprogress of an opponent by extending arm(s), shoulder(s), hip(s) or knee(s) orby bending his own body into other than a normal position or by using anyunreasonably rough tactics.
Art. 2. A player shall not contact an opponent with his hand unless suchcontact is only with the opponent’s hand while it is on the ball and isincidental to an attempt to play the ball.
Art. 3. A player shall not use his hand(s) on an opponent to inhibit thefreedom of movement of the opponent in any way
Art. 4.
The following acts constitute a foul when committed against a playerwith the ball:
a. Keeping a hand or forearm on an opponent;
b. Putting two hands on an opponent.
c. Continually jabbing an opponent by extending an arm(s) and placing ahand or forearm on the opponent;
d. Using an arm bar to impede the progress of a dribbler.
Art. 5. The following principles apply to post play and constitute fouls whenviolated:
a. Neither the offense or defense may dislodge an opponent during postplay by pushing, backing into, or using a leg, knee or arm to move anopponent from a legally established position.
b. A post player may establish post position with his arm(s) bent at theelbow. A straight arm may not be used to ward off an opponent or toprevent him from legally guarding the post player.
c. The “swim stroke†may not be used by the offense or defense toreposition, regain position, or move an opponent from a legallyestablished position;
d. Defensive players may use one arm bar on a post player with or withoutthe ball in the post area. The arm bar may not be extended or used todislodge an opponent.
e. Defensive players in the post are required to adhere to all other guardingrules."
http://www.ncaapublications.com/productdownloads/BR17.pdf