“Two entities are considered biologically separate if the following conditions are met:”

Unfortunately, this is a logically self-defeating question. The question assumes that there are two entities to begin with. You cannot then ask for a further criterion for what makes them different. The moment you look for the qualities of one entity and compare them to the qualities of the second entity, you have already proved that there are two entities. Any list of differences you discover in the two entities becomes a moot point.

Therefore (following Bertrand Russell), is there a way to rephrase this question to achieve the results you want? How about: “What makes you think there are two entities here, and not one?” If you accept that question, I’ll answer it. My answer is that:

1. The second entity – the fetus – develops into a unique and different person. Not sometimes, but all the time. No human fetus grows into anything non-human.
2. Unlike other entities of the mother’s body, or other cell configurations, the fetus does absolutely nothing biological on behalf of the mother. There are never any biological conditions within the mother in which her body’s response is “grow a fetus.” The fetus is not a biological response to anything. It has no biological function on behalf of the mother. It is entirely a non-mother-oriented entity.
3. At its earliest stages, the mother’s body supplies the development work, but that work is on behalf of the fetus, not the mother.

I’ll pause here, since I imagine there’s plenty of room for discussion of just those points.