Reference for Eruvin 186:5

אי הכי אימא סיפא פחות מכאן מערבין אחד ואין מערבין שנים אי בעיא חד תערב אי בעיא תרי תערב

If so, [should not the tenants of] the lower [courtyard]<span class="x" onmousemove="('comment',' Who can see a valid partition between their courtyard and the upper one.');">¹²</span> prepare an 'erub<span class="x" onmousemove="('comment',' For themselves only.');">¹³</span> [as in the case of] two [separated courtyards] but not a single one,<span class="x" onmousemove="('comment',' For the two jointly.');">¹⁴</span>

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