Well the line for Demi is that the two would both be incoming sophomores at Seattle University who decided to live in the dorms again, and randomly get assigned to each other. Jacey is really bubbly and friendly, but she's also really competitive. They wouldn't know how to feel about each other and maybe not get along at first, but then as the year starts to progress, they would find more in common with each other.
The line for Miley is that Miley would have been her best friend all four years of high school. They would love each other but it would seem like they hated each other because they compete for everything. Jacey got a 4.0, so Miley got a car. Miley made cheerleading captain, Jacey was senior class president. And just things like that. So they would still be best friends now, but still have that dynamic. When they're not competing for things, they're just like two normal best friends, hanging out and going to parties, getting ice cream and going to the movies, etc, etc.