My understanding of your question is that you currently have two files with their own "local" toctrees (which doesn't do anything incidentally) and want a third document providing a global toctree. If that's correct, then you just need to create that file, give it a toctree and specify which documents to link to:
.. toctree::
:maxdepth: 2
file1
file2
Note that Sphinx cares about the level of the titles, not really the document themselves, so because all your titles are "toplevel" your global toctree will be
not e.g.
I don't know which one you want, but if it's the latter you need to add a single top-level "document name" title to your document (customarily a title both overlined and underlined, the overlining effort is not huge since it's only a single title / line which is marked that way).