Mishnah Mikvaos 6:8: Mikvaot can be purified [i.e. made valid], a higher one by a lower one, and a distant one by a near one. How so? One brings a pipe of earthenware or lead [and puts one end in the upper mikveh], and puts his hand under [the other end of] it until it fills with water, and he brings it and touches it [to the lower and valid mikveh] even by a hair's width; that is sufficient [to render the upper mikveh valid]. If the upper [mikveh] contained forty se'ah [of valid waters], and the lower one had nothing, one can fill [water] by one's shoulder [i.e. drawn water which one carried over] and add to the upper one, until forty se'ah flow down into the lower one.

מְטַהֲרִים אֶת הַמִּקְוָאוֹת, הָעֶלְיוֹן מִן הַתַּחְתּוֹן, וְהָרָחוֹק מִן הַקָּרוֹב. כֵּיצַד. מֵבִיא סִלּוֹן שֶׁל חֶרֶס אוֹ שֶׁל אֲבָר, וּמַנִּיחַ יָדוֹ תַּחְתָּיו, עַד שֶׁהוּא מִתְמַלֵּא מַיִם, וּמוֹשְׁכוֹ וּמַשִּׁיקוֹ, אֲפִלּוּ כַשַּׂעֲרָה, דַּיּוֹ. הָיָה בָעֶלְיוֹן אַרְבָּעִים סְאָה וּבַתַּחְתּוֹן אֵין כְּלוּם, מְמַלֵּא בַכָּתֵף וְנוֹתֵן לָעֶלְיוֹן, עַד שֶׁיֵּרְדוּ לַתַּחְתּוֹן אַרְבָּעִים סְאָה:

Mishnah Mikvaos 6:9: Regarding a wall [dividing] between two mikvaot [which only contain forty se'ah 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].

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: