Memra of Shmuel in Bekhorot 46b

In Bekhorot 46b regarding the Memra of Shmuel, Rashi states the case is of twins, one of them a nefel (8 month baby), the other a full term (9 month baby). What in the sugya forces Rashi to interpret that one is 8 month and the other 9 month? It seems to me the case would also work if both were 8 month babies?

Since in those days an eight month baby didn’t have a chance of survival (Which is why he is called Nefel), the living baby must be of nine months, and the dead one - of eight.