1. Question: According to the Ra”n and Rema (Orach Chaim 38:8) who says if one is involved in a mitzvah he is exempt from another mitzvah even if you can fulfill both of them then why should one stop and say kedusha if he is in the middle of reciting the Shema?
A. The Shulchan Aruch (Orach Chaim 66:1) poskins that one can pause in reciting the Shema to say shalom to a person passing by who he reveres, for example his father or rebbe.
Answer: If one has an obligation to say shalom to a person he fears all the more so he should recite kedusha if the minyan is up to there in order to give honor to Hashem. The Mishna Berura there (19) says that if one pauses to say hello to a person all the more so for the sake of honoring Hashem. Therefore it has nothing to do with the concept of being involved in one mitzvah exempts one from another mitzvah.