There is a medical condition known as “Uterus didelphys”. Wikipedia describes it as follows:
Uterus didelphys (sometimes also uterus didelphis) represents a uterine malformation where the uterus is present as a paired organ as the embryogenetic fusion of the mullerian ducts failed to occur. As a result there is a double uterus with two separate cervices, and often a double vagina as well. Each uterus has a single horn linked to the ipsilateral fallopian tube that faces its ovary.
This is understandably an extremely rare thing. However this raises the question, what would be the din in regards to pidyon haben. If a woman gave birth to a boy from one womb then at a different time gave birth to a boy that formed in the other womb, would the 2nd require a pidyon haben?
Although this is a quite bizarre question, and it certainly won’t be documented anywhere, I believe the answer will be the same as a baby born by natural birth after a mother already gave birth via cesarean section.
In this case, the halachah is that the second baby is not considered a bechor (see Shulchan Aruch, Yoreh De’ah 305:24; based on Bechoros 47b; Rambam Nachalos 2:11 and Bikkurim 11:16; see also Rabbi Akiva Eiger in glosses on the Shulchan Aruch).
Similarly, although the second child born to the “other” uterus is a peter rechem, he will not obligated in Pidyon Haben. However, if the first baby turns out to be a nefel (see Rabbi Akiva Eiger there) it seems that the second baby, born from the other uterus, will be a Bechor.