Reference for Bava Batra 253:10

לאפוקי מדדרש רבא דדרש רבא שתי נשים שילדו ב' זכרים במחבא כותבין הרשאה זה לזה

gave birth [respectively] to two male children in a hiding place.<span class="x" onmousemove="('comment',' So that it is not known is who was born first. ');">³³</span> [these<span class="x" onmousemove="('comment',' When they came to claim a share in their father's bequeathed estate. ');">³⁴</span> may] write out an authorisation for one another.<span class="x" onmousemove="('comment',' Since one of the to is is certainly firstborn, he who receives the authorisation can claim from his brothers the double portion of the birthright, either on his own behalf or on behalf of his brother. The second clause of the cited Baraitha proves that Scripture did not permit of such a Procedure, and that in any doubtful case the double portion of the birthright cannot he claimed. ');">³⁵</span> R. Papa said to Raba: Surely Rabin had sent [a message stating]: This question I have asked of all my teachers, but they told me nothing; the following, however, was reported in the name R. Jannai: [If] they<span class="x" onmousemove="('comment',' The two sons of whom it is not known which is the firstborn. ');">³⁶</span>

Jastrow