Responsa for Bekhorot 97:6

תנא רב הונא

A MALE AND A FEMALE THE FATHERS ARE EXEMPT etc. R'Huna learnt: If they gave birth to two males and a female [in a hiding place and the children became mixed], the priest receives nothing.<span class="x" onmousemove="('comment',' The case of two women who had never given birth before married to two men is another instance of where the priest receives nothing. And although one child is a first-born in any case, for if one woman gave birth to two males, then one of them is a first-born and if one woman gave birth to a male and a female and the other gave birth to a male alone this one would be a first-born, nevertheless the son is not bound to redeem himself, for he can say to the priest: 'Perhaps I am not a first-born but the other'.');">⁶</span>

Teshuvot Maharam