Hello.
"Rashi - if a second fetus is conceived; Tosfos - this is impossible. Rather, we are concerned lest there are already two fetuses inside, and semen will enter between them"
After rejecting Rashi what would be the problem if "semen will enter between them"?
Thanks and best regards,
Richard Preschel, Paris, France
According to Rabeinu Tam (in Tosfos), if there are two (or more) fetuses in the womb, it is so tight that even the Shichvas Zera can put pressure on the delicate fetuses, causing one of them to deform.
Best wishes,
Mordecai Kornfeld