Create a new blank project. Set the start date of the new project to the earliest of the two projects. On the Project ribbon, click “Subproject” and select the first project to insert. Make sure the “link to project” box is checked. Repeat the steps with the second project. You now have a (temporary) master project with both subprojects inserted.
Expand to show all tasks. Click the Predecessor task in one project, press and hold the CTRL key and select the successor task in the second project. Release the CTRL key and click the Link tasks button. Repeat for other cross project dependencies.
Close the master (no reason to save) and save each inserted subproject when prompted.