Reference for Mikvaot 6:9
כֹּתֶל שֶׁבֵּין שְׁתֵּי מִקְוָאוֹת שֶׁנִּסְדַּק לַשְּׁתִי, מִצְטָרֵף. וְלָעֵרֶב, אֵין מִצְטָרֵף, עַד שֶׁיְּהֵא בְמָקוֹם אֶחָד כִּשְׁפוֹפֶרֶת הַנּוֹד. רַבִּי יְהוּדָה אוֹמֵר, חִלּוּף הַדְּבָרִים. נִפְרְצוּ זֶה בְתוֹךְ זֶה, עַל רוּם, כִּקְלִפַּת הַשּׁוּם. וְעַל רֹחַב, כִּשְׁפוֹפֶרֶת הַנּוֹד:
Regarding a wall [dividing] between two <i>mikvaot</i> [which only contain forty <i>se'ah</i> if combined], if it [the wall] was split along its warp [i.e. height-wise, from top to bottom], it is [considered] joined together [and both are valid]; if along its woof [i.e. lengthwise, from side to side], it is not [considered] combined, unless there is [at least] one place [where there is an opening] the size of the tube of a flask. Rabbi Yehuda says: the matter is reversed. If they [the waters] breached into one another [i.e if the was top of the wall was breached and waters mix together there, a breach] the height of a garlic peel and the width of the tube of a flask [is sufficient for them to be considered combined].