Sources of minhagim - kedusha and kri'as shema

1) What is the source of the practice of some people to turn left and right when saying the words "v'kara zeh el zeh v'amar" in kedusha?

2) What is the source of the hakpada of some people not to interrupt between the 2nd bracha before kri'as sh'ma and sh'ma, even by saying amein to that bracha (to the extent that they make it a point to conclude the paragraph at the same time as the shatz)? According to the accepted opinion of R' Yehudah in Brachos 2:1, one is even allowed to respond to a greeting between those two passages ("meishiv shalom l'chol adam") - so is there anything more to this practice than hyperstringency (or hyperleniency on saying this amein, if you'd rather).

Update, 6/29: It turns out that the p'sak of the Shulchan Aruch is that one is not allowed to interrupt between the 2nd bracha before Shema and Shema - the Shulchan Aruch HaRav explains that it's similar to any other bracha before a mitzvah. The Darchei Moshe, though, argues with this, being that the brachos of kri'as shema are not traditional pre-mitzvah brachos, as evident from the lack of the usual formula of Asher Kid'shanu, etc. I'm not sure how to fit the above mentioned mishnah into this shita. At least I'm an iota less ignorant now, though.