Hilchot Shabbat 11:11-12 (Types of writing for which a person is exempt)

If a person writes one letter on one page of a notebook, and another letter on a separate page of that notebook where the two letters cannot be read together, he is exempt. On the other hand, if he writes a letter in one scroll, and then writes a second letter in a different scroll in a completely different location, he is liable. Why is this so?!