Mishnah Berurah — משנה ברורה

Makes an interruption between them - Meaning, a long interruption which is in the category of "removal of thought" (ie. taking ones mind off of it. But [regarding] a short interruption, the Magen Avraham and the other later halachic authorities ruled later in Siman 206 that not speaking is not considered an interruption after the fact... even if he waited much longer than [the amount of time called] "כדי דיבור" (the amount of time it take to say "shalom alecha rebbi"). And [regarding] if he spoke between them... if his speaking was for the sake/purpose of the wearing [the ציצית]... for example, if he said: "Bring me a [ציצית] garment to wear" or [something] similar... then it's obvious that he does not need to go back and bless again. But if [he spoke] not for the sake of the wearing, there are [varying] opinions amongst the halachic authorities. The opinion of the Mechaber (Shulchan Aruch) later in Se'if 13 and the Magen Avraham and many of the later authorities [see the Shach - Yoreh Deah in Siman 19, Se'if Katan 7] are like the halachic authorities [that hold] that any speaking is in the category of interruption regarding this. But there are some later authorities that rule that since there some that hold regarding this that one does not need to go back and bless, that it is like one who spoke in the middle of a meal... that it's better that one not enter [into a situation] of a doubtful blessing (ie. there's a doubt if the blessing needs to be made again). And the Pri Megadim concurs with this. Therefore, every man should be careful to not speak (between the ציצית). And if he did speak, he does not go back and bless again. And certainly he should careful to not make a long interruption... for it is prohibited to cause [a situation where one will need to make] a blessing that is not necessary... And this is [as is seen] later in Siman 215.