Using the Options tab within the Views drop down on your project, you can select the Cross Project Dependencies. This function sets tasks from other projects in the system as available predecessors to this project's tasks.
Select the other project from the drop down list and select Add, then Save. Once completed you will be able to see the other project and its tasks for use.
Cross Project Dependency Rules
Example PATA -> PBTA -> PCTA -> PATB (P=Project, T=Task):
1. When Project B Task A is deleted, Project C Task A is connected to Project A Task A.
2. When project B is entirely deleted, Project C and Project A are not automatically calculated, but will recalculate the next time something changes. If the single task is deleted, then the recalculation does occur normally (if selected).
3. Although Project Insight checks for circular references, they should be avoided. The example above will cause the calculation of the schedules to take longer since we have to calculate all 3 projects at once each time any of them change.
4. When a project is copied, its relationships to external projects are not carried with it. But the dates of tasks, if pushed by those external relationships will maintain the dates that were last set.
5. When a project is deleted, successor projects may not automatically be calculated as well. The first time the project is calculated, then the dates may shift.