The Doubles Game

40. Playing the Ball in Doubles
The ball shall be struck alternately by one or other player of the opposing pairs, and if a player touches the ball in play with his racket in contravention of this Rule, his opponents win the point.

ITF Note 1: Except where otherwise stated, every reference in these Rules to the masculine includes the feminine gender.
ITF Note 2: See Rule 26(b) with regard to the Optional Alternative Scoring System in Doubles and Mixed.

USTA Comment: The partners themselves do not have to "alternate" in making returns. In the course of making one return, only one member of a doubles team may hit the ball. If both of them hit the ball, either simultaneously or consecutively, it is an illegal return. Mere clashing of rackets does not make a return illegal unless it is clear that more than one racket touched the ball.