In this step A and B divide the item into two fair shares, say they used the Divder-Chooser method. Let's say A got piece a and B got piece b.
Both A and B divide their shares into three "equal" shares each. Lets say
a-1, a-2, and a-3 are the pieces from A's division and b-1, b-2, and b-3 are the pieces from that of B.
Then C, our lone chooser, is asked to choose a piece from A's three pieces and a piece from B's three pieces and we are done.
Note: This method is not envy-free. Both A and B might envy C's share.