Sarah Chalke records the vocals for Beth and Space Beth separately because the characters are different, even though one of them is a clone of the other. "There is just a different texture to both of them," Chalke said to the press (via CBR). "They're coming from such a different place, and there's so much more confidence with Space Beth."
Where Space Beth is confident, the original Beth isn't even comfortable making the decision to stay with her family or leave. This is why she turns to Rick in the first place, thus leading to the creation of another Beth. Furthermore, just because there is a Beth clone, that does not mean the two are the same. Since the clone was made, their lives have gone in different directions, which affects who they are and how Chalke portrays them. Fans also can't forget that the whole purpose of a clone was for Beth to find herself, and Space Beth's confidence exemplifies that, regardless of if she is the original Beth or the clone.
While Beth is still Beth, Chalke is right. The Beths are different characters because they are different versions of themselves. Where one is confident, the other still struggles to figure that out for herself. To keep that straight, it makes sense that the crew would record each Beth one at a time to avoid confusion.
