As long as it doesn't cause distractions to your opponents during a rally...... . As a coach, I have always encouraged trainees to communicate to each other during a rally, when playing Doubles. As long as it doesn't cause distraction to your opponents; it is allowed. .
Talking should be fine, but if you are distracting your opponent, I.e. Shouting when they are about to hit a shot is wrong, but communicating to your partner to get a shot is fine.
I agree with both of the two above. If the opponent is about to strike the shuttle, you cannot shout until they have hit the shuttle. But shouting to each other apart from that is fine.