א מִפְּנֵי שֶׁצָּרִיךְ לְדַקְדֵּק בְּשִׁעוּר הָעִסָה שֶׁלֹּא לְהַרְבּוֹת בָּהּ מִשּׁוּם חֲשַׁשׁ חִמּוּץ, וּמוּטָב שֶׁיְּמַעֵט בָּהּ, לָכֵן טוֹב לְקָרֵב הָעִסוֹת יַחַד בִּשְׁעַת הַפְרָשַׁת חַלָּה שֶׁיַּשִּׁיקוּ זוֹ בְּזוֹ, דְּשֶׁמָּא יֵשׁ בָּהֶם (אַחַת) שֶׁלֹּא הָיָה בָּהּ כְּשִׁעוּר. וְאִם אִי אֶפְשָׁר לְהַפְרִישׁ חַלָּה בְּעוֹדָהּ עִסָה מִפְּנֵי הַמְּהִירוּת, יַפְרִישֶׁנָּה אַחַר אֲפִיָּה מִיָּד, שֶׁיִּתֵּן כָּל הַמַּצּוֹת בַּסַל וְהַסַל מְצָרְפָם לְחַלָּה; וְזֶהוּ הַדֶּרֶךְ הַיּוֹתֵר נָכוֹן. הַגָּה: וְאִם הָיָה בּוֹ שִׁעוּר חַלָּה וְשָׁכַח לְהַפְרִישׁ, יַפְרִישׁ אַחַר כָּךְ וַאֲפִלּוּ צֵרוּף סַל לֹא צָרִיךְ. וְאִם לָקַח מִמִּקְצָת הָעִסּוֹת, וּמִמִּקְצָת לֹא לָקַח, וְנִתְעָרְבוּ, צָרִיךְ לִקַּח חַלָּה מִכָּל אַחַת וְאַחַת אוֹ יָלוּשׁ עִסָּה אַחֶרֶת וְיִקַּח מִמֶּנּוּ גַּם כֵּן עַל אוֹתָן שֶׁנִּתְעָרְבוּ. וְאִם מַכִּיר מַצָּה אַחַת שֶׁחַיֶּבֶת בְּחַלָּה, נוֹטֵל מִמֶּנָּה עַל הָאֲחֵרוֹת (הַגָּהוֹת מַיְמוֹנִי פ"ג וּמָרְדְּכַי ר"פ אֵלּוּ עוֹבְרִין וּבֵית יוֹסֵף בְּשֵׁם תּוֹסָפוֹת פ"ק דְּבֵיצָה).
1 THE LAWS ABOUT THE PRACTICE OF SEPARATING CHALLAH TITHES FROM MATZAH DOUGH Because we need to be particular that the proportion of dough is not too large, as there is a concern that the dough may leaven, it is good practice that it [the dough proportion] be lessened somewhat. To that extent, it's good to converge the doughs together at the time of separating Challah tithe to the extent that the doughs kiss[adhere to] each other in a case where they individually don't have the proportions [to be obligated to tithe]. If it's impossible to separate Challah while still in the dough phase because of [leavening] time constraints, it should be separated immediately after baking, when all the Matzot have been placed in a basket. The basket serves to incorporate them together [to meet the proportion for seperating] Challah tithe, and this is the most proper way to do it. Rema: and if there is the proportion for Challah tithe and one forgot to separate, one should separate after that and even incorporating them with a basket is not necessary. If it was taken from some of the doughs and from some of the doughs it was not taken, and they became mixed, it is necessary to separate from every single one [made from that dough] or to knead an alternative dough and take from the alternative on account of the mixture. If it's recognized that one of the Matzot [from the mixture] is obligated in Challah tithe, he should take from it on account of the others. (Haghaot Maimoni perek 3 and Mordechai in the beginning of perek "Ayloo Overeen" and B"Y in the name of Tosafot of the first perek of Beitzah)