Ken, if you have a master project with a sub project then the predecessors are pretty simple. This means that they both are viewable in the same schedule. Just select the predecessor in one project and the successor in the other project and then select to link the tasks (Task ribbon, Schedule group, Link).
It is still possible as separate project schedules. Easiest way to link tasks between projects is to create a temporary master project. From a blank project, select the Project ribbon, Subproject. Find the first project and insert it. Do the same for the second project. Now you have both in the same schedule, and follow the steps above to link tasks. After that, close that project, select No to save changes, and the Yes To All to save changes to the sub projects.
There are a few considerations with this and may be a bit challenging to a beginner, but at least worth testing to start. If this is too much I might suggest combining the two schedules. That’s the simplest solution.