It might be difficult to distinguish tell the difference between the wroting of one person and of two people, one of whom is trying to imitate the other. If two people were selected at random, however, and asked to write the same text without being given any model to copy from, it is unlikely (though not impossible) that they would look even remotely alike. If ten people are selected at random, the likelihood of all ten having similar handwriting would start to become astronomical unless all samples of handwriting were linked to some logical common model (e.g. if all the people have just finished a penmanship class).
If all samples of handwriting precisely match those of a third-grade cursive text, that would not necessarily imply they were all written by the same person (though it would seem odd). If all samples, however, have an unusual trait, that would be far more incriminating. It would still not prove that they were all written by one person, but it would strongly suggest that they weren't all written by different people.