Mishnah Berurah — משנה ברורה

There is no concern for etc. - Because [regarding] the ציצית that remained [on each half], they are not considered invalid since they (ie. the garments halves that the ציצית are affixed to) have the required measurement to wrap in. Therefore, if he wants, he may join to that half, a piece of another garment, and make/tie ציצית on the new corners (ie. the 2 corners from the newly adjoined piece of garment). And certainly he may make a complete טלית from [either of] the halves [that ripped from the original טלית], meaning that he can make/tie ציצית on the new corners (ie. the new corners that it now has after it ripped in half). And if he wants to re-attach the two halves that separated on from the other, there are those who say that he has to first untie the ציצית from one of the halves, and then sew [the two halves], and after he sews them he should affix the ציצית onto it; because if he does not [first] untie them, it's in the category of "Make it, and not from that which is [already] made". But the majority of the later rabbinic authorities are lenient and hold that this is not in the category of "Make it, and not from that which is [already] made", but nevertheless it is good to be stringent optimally.