I love Cross-Project predecessors! It is a great way to connect various projects that are dependent upon each other.
Here is the article from the support site that walks you through the steps:
https://support.workfront.com/hc/en-us/articles/216741968-Cross-Project-Predecessor
One thing to note though: timelines only recalculate when something changes on that project. So if your cross-project predecessor changed, project 2 won't automatically upon. I normally highlight all my projects, edit, and hit the recalculate timeline option in the morning, and when I get back from lunch just in case things on my projects need to move based on other people's projects.