Ibn Hajar al-Haytami narrated that al-Qaadi said: Du’aa’ during this sitting (between the two khutbahs) will be answered. Then Ibn Hajar said:
It may be understood from what al-Qaadi said that the Sunnah is for the people present to make use of the time when the imam sits to say du’aa’, because it is affirmed that it is answered at that time. If they say du’aa’ then it is better for it to be silent, because by saying it out loud they may disturb others, and because saying it silently is better in the case of du’aa’, unless it is for a reason. End quote.
Al-Fataawa al-Fiqhiyyah al-Kubra by al-Haafiz Ibn Hajar al-Haytami (1/251-252).