Siman 412
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א עֵרֵב חֲצִי הַיּוֹם הָרִאשׁוֹן לְרוּחַ צָפוֹן, וְהַשֵּׁנִי לְרוּחַ דָּרוֹם שֶׁטָּעָה וְהָיָה סָבוּר שֶׁאֶפְשָׁר לַעֲשׂוֹת כֵּן, אוֹ שֶׁאָמַר לִשְׁנַיִם: עָרְבוּ עָלַי, אֶחָד עֵרֵב עָלָיו לַצָּפוֹן וְאֶחָד לַדָּרוֹם, אִם הִנִּיחַ כָּל אֶחָד עֵרוּבוֹ בְּסוֹף אַלְפַּיִם לֹא יָזוּז מִמְּקוֹמוֹ, שֶׁאֵין יָדוּעַ אֵיזֶה עֵרוּב קָנָה לוֹ, וְאִם לֹא נָתְנוּ הָעֵרוּבִים בְּסוֹף אַלְפַּיִם, הוֹלֵךְ מַה שֶּׁאֶפְשָׁר לוֹ לֵילֵךְ מִכֹּחַ שְׁנֵיהֶם. כֵּיצַד, נָתַן כָּל אֶחָד עֵרוּבוֹ לְסוֹף אֶלֶף מִבֵּיתוֹ, יֵשׁ לוֹ אֶלֶף אַמָּה מִבֵּיתוֹ לְכָל אֶחָד מֵהָעֵרוּבִים. נָתַן אֶחָד לַמִּזְרָח לְסוֹף אֶלֶף, וְהַשֵּׁנִי לַמַּעֲרָב לְסוֹף ת"ק, הוֹלֵךְ לַמַּעֲרָב אֶלֶף מִכֹּחַ מַה שֶׁשִּׁיֵּר לוֹ הָעֵרוּב שֶׁבַּמִּזְרָח, וּלְמִזְרָחוֹ אֶלֶף וְת"ק מִכֹּחַ מַה שֶׁשִּׁיֵּר לוֹ הָעֵרוּב שֶׁבַּמַּעֲרָב.
1 Laws of one who divides his eruv. Contains one section One who made an eruv for the first half of the day toward the north and the second half toward the south — thinking mistakenly that this is possible — or one who said to two people "make an eruv for me" and one made it for him toward the north and one toward the south: if each placed his eruv at the end of two thousand cubits, he may not move from his place since it is unknown which eruv took effect for him. If the eruvs were not placed at the end of two thousand cubits, he may walk as far as possible by virtue of both of them. How so? If each placed his eruv at the end of one thousand cubits from his home, he has one thousand cubits from his home toward each of the eruvs. If one placed his eruv to the east at the end of one thousand cubits and the other placed his to the west at the end of five hundred cubits, he may walk one thousand cubits to the west by virtue of what the eastern eruv left him, and one thousand five hundred cubits to the east by virtue of what the western eruv left him.