When you are playing doubles you must know if you or your team is in the offense, (you are receiving an easy ball from your opponents, i.e., a short ball, an easy volley or a shallow overhead), or in the defense (when you are near the net, but your opponents are attacking you with a high volley, an overhead, or a kill-shot, such as a shallow bouncing ball that your opponent attacks you with).
Typically, when you are in the offense you should attack the closest opponent to you, if they are in the up and back position. When you are in the offense and your opponents are both up, near the net, you should attack down the middle - forcing them to hit more up and cause confusion to who will take that shot. When you are in the offense and your opponents are both back, you are trying to attack the weaker player. Look to hit an angle volley or overhead, like aiming at the side pocket in pool - not the corner pocket.
When you are in the defense, being attacked, and your opponents are up and back, return the ball to the opponent who is the farthest away from you. When in the defense and your opponents are both up, try to defend the ball back low and down the middle. When you are in the defense and your opponents are both back, just push the ball back three quarter depth and start the point again.
Remember, your shot selection is completely controlled by you knowing if you are in the offense or defense and what position your opponents are in.
Good luck on the courts!