Why do some people say ‘Two people have a baby’ can mean ‘two people have a baby each’ but others say it can’t mean ‘two people have a baby each’ if there’s no ‘each’?
Two people have a baby.
A: It can mean two people have a baby each.
B: No, it can’t mean two people have a baby each unless you use ‘each’. It only mean two people have one baby together.
The problem (as others have pointed out) is that meaning is created via context. There are quite a few questions here that take the pattern "Here's a short sentence. Can it be taken to mean [A] or [B]?" and the answer often is "Well, surrounding context could sway the interpretation to either A or B, but the raw sentence in itself doesn't make it clear." You're looking for an absolute ruling on what a sentence "can mean," or absolutely "can not mean." This is tricky, since words are just placeholders for ideas, and we can bend them to our purposes; with enough perversity and preparation, any sentence can mean anything.
So the short answer to your question is, it can be used for either meaning, but it will do so only with the help of surrounding context. If we want one meaning or the other, it's up to the context to set it up and make it clear (which it might do well or poorly). If we want to talk about which meaning is most likely given only the single, raw sentence, we can, but it's a bit of an abstract exercise. If a stranger walked up to me on the street, looked me in the eye, said "Two people have a baby," and walked away, I wouldn't know what meaning to take from it.
In this case, the meaning is heavily influenced (as Edwin mentioned) by the fact that "have a baby" is an existing phrasal idiom, usually meaning "conceive and give birth to a baby," and is often applied to two people "together." If you changed the noun, you might lean toward one meaning or the other.
Two people have a coffee —Probably (not definitely) taken to mean that two people meet over a coffee apiece—again, "have a coffee" is an existing idiom.
Two people have a car —Probably (not definitely) taken to mean that two people share a car, since joint ownership of cars is common.
Two people have a plane ticket —Probably taken to mean that two people have a plan ticket each, since tickets are usually individual.
