סימן ט סעיף טו

טַלִּית שֶׁיֵּשׁ בּוֹ צִיצִית וְנֶחֱלַק לִשְׁנֵי חֲלָקִים, כְּמוֹ שֶׁהוּא שָׁכִיחַ בְּהַרְבֵּה טַלִּיתִים שֶׁלָּנוּ שֶׁהֵם מְחֻבָּרִים מִשְּׁנֵי חֲלָקִים, וְלִפְעָמִים מַפְרִידִין אוֹתוֹ כְּדֵי לְכַבְּסוֹ אוֹ לְתַקְּנוֹ וְאַחַר כָּךְ חוֹזְרִין וּמְחַבְּרִין אוֹתָן בִּתְפִירָה, כֵּיוָן דְּמִסְּתָמָא יֵשׁ בְּכָל חֵלֶק כְּדֵי לְהִתְעַטֵּף בּוֹ, עַל כֵּן דַּי אִם יִטֹּל שְׁתֵּי הַצִּיצִית מֵאֵיזֶה חֵלֶק שֶׁיִּרְצֶה. וּלְאַחַר שֶׁיְּחַבֵּר אֶת הַטַּלִּית, יַחֲזֹר וְיִקְשְׁרֵם. אֲבָל אִם אֵין בְּכָל חֵלֶק בִּפְנֵי עַצְמוֹ כְּדֵי לְהִתְעַטֵּף בּוֹ, צָרִיךְ לְהָסִיר כָּל הַצִּיצִית (מִשּׁוּם שֶׁכְּשֶׁנִּתְפָּרְדוּ הַחֲלָקִים, נִפְטַר כָּל חֵלֶק מִצִּיצִית. וּלְאַחַר שֶׁשּׁוּב נִתְחַבְּרוּ וְנִתְחַיָּב בְּצִיצִית, אִם יִשָּׁאֲרוּ בוֹ הָרִאשׁוֹנוֹת, פָּסוּל מִשּׁוּם תַּעֲשֶׂה וְלֹא מִן הֶעָשׂוּי כְּדִלְעֵיל סָעִיף ו). וְאִם בְּחֵלֶק אֶחָד יֶשׁ בּוֹ כְּדֵי עִטּוּף וּבְחֵלֶק אֶחָד אֵין בּוֹ כְּדֵי עִטּוּף, יָסִיר אֶת הַצִּיצִית מֵחֵלֶק זֶה שֶׁאֵין בּוֹ כְּדֵי עִטּוּף.

Please refer to preface to para. 3 where the rule of Ta'aseh velo min he'asui was discussed. Here we find another situation where the problem arises:
A large tallis with a seam down the middle whose tzitzis are validly attached is separated at the seam for the purpose of facilitating the handling of the tallis during laundering or mending. We, thus, have "two" taleisim (the Yiddish plural of tallis) with two tzitzis each. We are now faced with a problem—sewing the two halves together again would result in validating the tallis for the mitzvah through an act not done directly to the tzitzis. Thus: Ta'aseh velo min he'asui. The author here proposes a solution to the problem.
This paragraph deals with a tallis that had tzitzis [attached] and was divided into two [halves] as is common with many of our taleisim, that are sewn together from separate sections. Occasionally [the sections] are separated in order to launder it, or to mend it, and then is restitched. Since there is usually enough left in each section to enwrap a person, it is sufficient to remove two of the tzitzis from any section and after restitching the tallis to retie the tzitzis.⁴⁵Thus avoiding ta’aseh velo min he’asui. However, if each section is not large enough to enwrap a person, he must [first] remove all of the tzitzis (because when the sections were separated each part is exempt from the mitzvah of tzitzis, and when they are later reattached and require tzitzis, then, if the original tzitzis remain the tallis is invalid because of Ta'aseh velo min he'asui as [was mentioned] previously in paragraph six). If in one section there is a sufficient amount [left] to enwrap [a person], and in the other section there is not enough to enwrap [a person], he should [then] remove the tzitzis from the part that does not have enough to enwrap a person.

תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)