When there is more than one person saying kaddish, and they are not saying it in synchrony, who are we answering amen to?
There are a few opinions addressing this question. In the Mishnah Berurah Siman 55 it says that when two or three people say kaddish together and one goes ahead, if the difference is not more than two seconds, then you answer for either and it's like you answered both, but if the difference is more than two seconds, you should answer each one individually.
The second opinion is written in שערי תשובה, which says that a person should only answer to the first one.
The third opinion is from the אשל אברהם that says that a person should answer to the last one.
The fourth opinion is שולחן הטהור which says that a person should answer to the one standing closest to him and he can hear very well.
All agree that all of these opinions are בדיעבד. They all agree that לכתחילה all saying kaddish should try their best to say it in synchrony on order to not cause confusion, and it is highly recommended that they should all stand close to each other.
In a different Tshuva we discussed a situation where an Ashkenazi goes to a Sephardi shul and says his kaddish or vice versa, and we mentioned there the conclusion of harav Moshe Feinstein in his book Igrot Moshe, volume 2 Siman 24 which says that as long as a person is praying quietly to himself, he should follow his own custom, but once he does something public, he needs to respect the customs of the place he is praying at. Therefore, when one is saying kaddish, he should first make sure that he is saying the kaddish that this place is accustomed to.
Nevertheless, if he goes to a shul with a diversity of customs, he is allowed to say his own Ashkenazi kaddish even out loud, and he needs to be aware that his kaddish is shorter than the Sephardi kaddish and needs to read it slower than Ashkenazi kaddish.