How is one to answer Amen to Kaddish if more than one person is saying Kaddish and they are not saying it simultaneously?^[1]
If they do not conclude the stanzas in Kaddish simultaneously, and there is an interval of Kdei Dibbur between the two conclusions of the blessing, then one is to answer Amen for each conclusion of the stanza. If however they conclude within Kdei Dibbur of each other, then one can choose when to answer Amen, either with the first person concluding or the second person concluding, and the Amen counts for them all. Some Poskim^[2] however rule that if one person can be clearly heard while the second is at a distance and cannot be heard well then one is to answer to the person who is closest to him and who he heard clearly.

If one is hearing Kaddish from a few different Minyanim, and they are in different places of Kaddish, how is he to answer Amen?^[3]
One is to answer Amen only for the first person saying Kaddish.

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[1] Halachos Ketanos 2/48; Beir Heiytiv 55/1; Kesher Gudal 9/27; Shalmei Tzibur p. 87; Beis Oveid 8; M”B 55/4; Kaf Hachaim 56/8; See Piskeiy Teshuvos 55/6

Other opinions: Some Poskim rule one is to answer after the last person who concluded and it counts also for the first person. [Ashel Avraham Butchach 124/6] See Piskeiy Teshuvos ibid who writes that the Shaareiy Teshuvah 56/3 and Birkeiy Yosef 56/1 argue on the above and rule that one is to only answer Amen to the first person concluding. This is incorrect as the Birkeiy Yosef and Shaareiy Teshuvah were not discussing the above case but rather a case in which you have many different Minyanim taking place in the same room and you are unable to answer Amen for every Minyan’s Kaddish. The Kesher Gudal [author of Birkeiy Yosef] brings both Halachos, one in 9/27 and one in 9/28, thus proving they do not contradict each other but are rather speaking of two different cases. Likewise, the Kaf Hachaim 56/7-8 records the two rulings in two separate Halachos and referring to two different cases, thus proving it is not a dispute or contradiction.

[2] Shulchan Hatahor 132/4; Mishneh Sachir 11

[3] Birkeiy Yosef 56/1; Mahariy Malko 19; Kesher Gudal 9/28; Shalmei Tzibur p. 87; Beis Oveid 8; Shaareiy Teshuvah 56/3; Chesed Lealafim 56/6Ben Ish Chaiy Vayechi 11; Kaf Hachaim 56/8