spoilers
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In the world of the novel, members of different species can't organically reproduce, but there is a medical procedure that allows for it to happen. Same sex couples could also have biological offspring as a result of said procedure, and this procedure also has a high rate of multiples. From my understanding, the procedure works on an instance-per-instance basis rather than a permanent change that permanently allows one to bear hybrid species offspring. And who carries the pregnancy is completely independent of anything sex related.
When the puppy (apologies, forgot names) talks to the wolf about having children, the wolf heavily insists that he be the one to carry the pregnancy since he is worried about puppy's health and wellbeing. The puppy insists that he should carry since the wolf has an important job that should not be on hold for too long.
They come to a compromise, the puppy "agrees" to let the wolf carry first, the puppy carry second, and switch off as they have children. "Agrees" in quotes since the procedure has a high failure rate, and the doctor convinced them to undergo the procedure at the same time. The procedure fails for the wolf, but is successful on the puppy.
This results in a set of twins. Both mostly black wolf dogs, one female with a white spot on her chest, and one male with a white sock
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