By Yehuda Shurpin
Most surprising in this article is the finding that two famous Yekke's actually promoted - one temporarily - multiple people saying kaddish.
One of the first to mention the custom to have all mourners recite the Kaddish is Rabbi Yaakov Emden (1697–1776), who writes in his siddur that “with regard to various laws about who takes precedence for Mourner’s Kaddish among the Ashkenazim, I will not discuss it, as it is only a custom (and how good and right is the Sephardic custom that if there are many mourners, all merit and recite the Kaddish together, avoiding strife and disagreement) . . .”2 In other words, Rabbi Yaakov Emden felt that it would be wise for Ashkenazim to follow the Sephardic custom of reciting the Kaddish together and thereby avoiding strife.
Note from reader EA:
Rabbi Hamburger discusses this R. Emden approval of this minhag in his lecture found in YUtorah.org at minute 46 onwards: https://www.yutorah.org/lectures/lecture.cfm/789579/rabbi-binyamin-hamburger/the-development-of-kaddish-yasom/