The Shulhan Aruch writes that one should "run" to hear and respond to Kaddish. If one has the opportunity to hear the recitation of Kaddish, he should enthusiastically seize the opportunity. Sometimes, people are in a rush to leave the synagogue early, and they forfeit opportunities to hear Kaddish. Responding to Kaddish is a precious Misva, and so one should eagerly seize opportunities to do so.
If a person is in a place where two different Minyanim are occurring simultaneously – such as at the Kotel in Jerusalem – and he hears one Minyan reciting Kaddish, and another Minyan reciting Nakdishach, then he should respond "Yeheh Shemeh Rabba" at the expense of responding to Nakdishach. However, this applies only if the person does not need to fulfill his Nakdishach obligation at that time – meaning, he already recited that prayer, or he will be reciting that prayer in a Minyan later. But if a person is praying with a Minyan, and as the Hazzan reaches Nakdishach he hears Kaddish from a different Minyan, then he should respond to Nakdishach in the Minyan in which he is participating at that time. In this instance, his current prayer service takes precedence over the Kaddish being recited in a different Minyan.
If a person began responding "Yeheh Shemeh Rabba" to Kaddish, and at that moment he hears Nakdishach, then he should end his response after "Almaya Yitbarach" so he can respond to Nakdishach Normally, our custom is to extend our response of "Yeheh Shemeh Rabba" through "Be'alma." In this case, however, in the interest of being able to respond to Nakdishach, one should end his response with "Almaya Yitbarach."
If one hears Kaddish while listening to Nakdishach, he should respond to Nakdishach as usual without interrupting to respond to Kaddish.
If a person finds himself near two Minyanim, one of which is about to recite Kaddish Titkabal (the Kaddish recited after the Amida) and the other is about to recite Nakdishach, then he should join the Minyan that is about to recite Nakdishach. The reason is, quite simply, that the Minyan which is now starting Nakdishach will recite Kaddish Titkabal after the repetition of the Amida. Therefore, by going to that Minyan, one has the opportunity to hear both Nakdishach and Kaddish Titkabal. This is the ruling of the Mishna Berura. The Kaf Ha'haim (Rav Yaakob Haim Sofer, Baghdad-Jerusalem, 1870-1939) adds that this applies even if one must pass by the Minyan reciting Kaddish to get to the Minyan reciting Nakdishach. Whereas normally it is improper to pass by a Misva opportunity, in this instance it is preferable to go to the further Minyan for the reason discussed.