Can you have two separate schedules that are linked together?

It is done at the activity level in relationships. To assign a relationship to an activity outside of the current project, in the Assign Predecessors dialog box click the Display Options bar and choose Select Project. Select the external project, then select the predecessor activity you want to assign. Click the Assign button then click the Close button. See my screen shots below for how the process is handled in Primavera. The project data below are from different copies of the PPS projects. Not the actual project.